I think there are a lot of different possibilities here.
Maybe there was genuine chemistry between the two of them. But I also think attraction like that CAN be one sided, and we can fool ourselves the other person feels it too, just because we want them to.
I have felt that sort of attraction for a number of people in my life - I am highly sexed and a bit of a romantic and fall in love easily.
It happened more often when I was young. And the most recent one was my current partner. We met on an internet date, and the attraction, at least on my side, was like being hit by a steam train - I could hardly talk to him at first - after our first date i couldn't sleep or think of anything else but going to bed with him. And when we shagged on the second date, yes it was even more mind-blowing than I imagined.
Four years on and I can still feel like that for him, though also there has been a lot of water under the bridge, and my feelings are just pure unbridled lust and infatuation any more ... But the intensity of the attraction - that has definitely been a glue to hold us together through trickier times, and I wouldn't be without it.
OP, you can't know whether this bloke feels the same unless you ask him. And in all decency, you can't do that while you are still with your partner.
What is your current relationship like. Does it tick your boxes, or is it lacking? I would try and make a decision on this quote aside from your chemistry with this new bloke, if you can....
And if you DO decide your current relationship has run it's course, that is the time to consider whether it is worth having a go with Mr Chemistry. (If he wants to of course - you can't know for sure until you ask him. And you should only act on it if he is willing to move on from his LTR too. Lots of variables here.....)