Posting because I was once in your shoes a few years back now (at least it sounds very familiar reading your posts), was seeing this guy on and off for over 10 years since we were teenagers and my brain kept being like 'it can't just be coincidence that we keep ending up back together, even if it's only briefly each time, it must be fate!'. This guy also moved in with someone else for a while and I felt so crushed like 'if only he'd given me a real chance'. Then it was even worse, because I hadn't spoken to him in years whilst he was with that woman, but then he told me it had been my name that had been thrown around somehow during their breakup (at least, that's his story🙄). I felt all this misplaced hope that maybe this time we would be together for real. Only that never happened, again. I thought I was so in love with him, I thought I could never do better, sexual attraction off the charts, everything. It was overwhelming for a very long time. I feel so differently about it all now, feels weird to even remember that was genuinely how I felt.
Therapy helped me loads to realise he had let me down big time, not just once, but over and over again, all under the guise of 'well we're not really together'. Only to go back to leading me on emotionally again. It also helped me not to beat myself up for doing that, because I had a whole heap of crap I was dealing with throughout that time which meant I never had time to come up for air and actually think deeply about what was going on relationship-wise. I think it's too easy to fall for these elusive guys (especially if you met him when you were relatively young like I did) when you never actually really get to experience an 'actual relationship' with them. Your brain keeps telling you 'but what if we finally made a real go of it, we've never actually done that, then it might be perfect!'. The truth is, you probably have been experiencing exactly what he is like 'for real' all along, you've just not had the official titles because he's somehow always found an excuse to not do that. Just for whatever reason, he's decided to give the title thing a try with his current partner and even move in with them. That doesn't mean he's suddenly matured and/or is now completely wonderful and/or you 'missed your chance'.
Frankly, heaven help her, if I was her and I heard that he a) found out that his ex was working in the same place he was applying to and then b) immediately met her to have a private coffee so it 'wasn't weird' when you met at work again, there would be some serious conversations taking place. I know you say you work in a niche industry, but is it really so very niche that he couldn't give you both the dignity of just staying out of your life when he found out you were working there and applying somewhere else, without contacting you? Surely there are alternatives, they're probably just more inconvenient for him. I know I wouldn't want to be working with any of my previous exes by choice, even if it meant going somewhere miles away. Especially, as you seem to have made it clear to him, you still are attracted to him. He's potentially putting so much at risk by doing this (his job, your job, his place to live, his current relationship, etc.). This guy just seems like trouble, and not in the 'ooh fun trouble' way, the 'my life and everyone else's around me is ruined' sort of way.
So sorry he's being frankly, quite selfish. Maybe deeply consider if this 'peace offering' to meet for coffee was really anything to do with your feelings, or more to do with him wanting your go ahead for him to muscle in on a workplace you're already established in, when he knows you two have history? Because it might be worth you seriously considering whether it's worth you telling him now, before he starts, if you don't think you two could comfortably work together (and I highly doubt you can if you're still lusting after him but he's literally living with someone else. Especially is he's going to send such ridiculous, vaguely-flirty texts like 'maybe late fate decide' when you're saying you don't know how you're not going to go mad with him being there). It's either that, or you've got to hope he fluffs it somehow in the first few weeks and gets sacked, or you will be the one having to switch jobs to whichever inconvenient place he would have gone to in the first place if he wasn't trying his luck. I know it's hard when your hormones are going nuts and you're just imagining this perfect life with this guy who can occasionally make you laugh, but really do trust me that I've since met a man who does all these things better and without the horrific heartache and pain that comes with the self-centered behaviour of a man like the one you're crazy about right now. I really think you won't be crazy about him forever.
Stay strong, cry all you need to, take time to look after yourself and really reflect on if this relationship is actually good for you. Imo, you need to do whatever you can to continue keeping this man out of your life.