Stripping away all the doubts this has set in your mind what has actually happened?
A person decided to hide behind a fake name that they knew you wouldn't recognise.
They then said something but crucially nothing outright, nor something you can easily confirm about an ex partner from over 10 years ago. A ex partner who confirms your partners story about an amicable split.
This person then refused to answer your questions and even went so far as to block you, then to create a extra level of doubt decided to unblock you.
Your partner handed over his phone when you asked, did he make a fuss about it? Did you find anything unusual?
The person who is most messing you about right now is the unknown one and yet they have managed to make you doubt someone you have known for 5 years. Do they know your past hurts and how to twist the knife in for you?
Anyone with decency and real principles would have confronted your partner about cheating and asked him to confess to you before telling you, in plain sight and full knowledge told you if he refused. If he has done something. Instead they created a poison pen letter situation with you, not outright saying anything but just enough to cause harm, creating doubt to hurt you and ruin your relationship with your partner for reasons you will probably never know. Hardly the act of a sound mind or someone with principles is it.
Like you I would be guarded now, it would be only natural but as much as it could be something your partner has or hasn't done at any one point, it could equally be about hurting you in a really unpleasant way.
When we receive messages from the kinds of nasty vile mind that create poisoned pens, it doesn't have to be true or have anything to do with anyones past. It just has to mention a past event or person and designed to create maximum doubt and unhappiness for their own personal thrill. It's just about hurting people for twisted and no good reasons.