Is low-formaldehyde plywood important if the wood will be finished with polyurethane?
No, not in terms of the finishing at least. It's important if you want to make stuff that has low VOC emissions of course but doesn't appear to make any difference to how a finish dries, cures or maintains over time which is not surprising given the very slow outgassing of the formaldehyde.
FYI, "polyurethane" isn't enough of a description if you don't go on to clarify the exact type of product you used, as you did in the body of the Question. This is because there are multiples types of polyurethane finish and they are very different indeed. Waterbased polys are actually nothing like 'real' polyurethane, oil-based polyurethane varnish. They are acrylic-suspension finishes with a little added polyurethane, and are not varnish in the true sense of the word despite being called that by many manufacturers. Basically the same product might be called "acrylic lacquer" by another maker and they name would be just as accurate.
Can hazardous gasses escape the polyurethane layer?
In short yes. Coat thickness is a factor but essentially water vapour can pass through almost any finish so they offer no real barrier to gases. Finishes slow the rate of exchange but don't stop it.
And as mentioned above waterbased poly is mainly acrylic and this type of acrylic finish can be surprisingly porous in nature precisely because it starts off as a suspension in its liquid form (tiny droplets surrounded by water).