Iz stated that Mark knows who Jack is, and we've seen him already.

Not just that we've seen him. Mark knows this person.

Now, for this we're going to assume Mark was at Timmy's birthday party. something that, let's be honest, is basically confirmed.

now who would be at Timmy's birthday party? his parents. more specifically, Jason. Timmy's father.

So why does Mark not instantly recognize Jason or even mention anything?

because Jason isn't his real name. Jason's real name is jack.

they are one in the same.

After the incident Jack would've probably been suspect #1 for what happened. so to avoid the cops finding him and other people recognizing him, he changed his name to Jason baker.