I heard the same rationale; plus, some other details: apparently the Poles were looking for a way to recycle/pass on those MiG aircraft, and came up with a plan to offer them to the US in exchange for buying more modern fighter jets from the US--dangling a deal that served their own interests.

A criticism I heard of their move is that it popped up out of nowhere, unilaterally, instead of being discussed in the talks that have been taking place among the EU/NATO countries.

If they wanted to freely donate them to Ukraine and fly them there themselves or get Ukrainian pilots into Poland to fly them, they could easily do it. It's the country next door after all.