There are about 40 entrants listed from Ukraine. Most of them are from Kyiv, and a few are from even worse off places like Kharkiv and Kherson.
I doubt many of them will make it to Boston. Some of them may be fighting to protect their homeland, or may have had their homes blown to smithereens. Most of them have loved ones who are in danger or even dead. Those who do make it will need to spend days getting to and crossing the Polish border amid checkpoints and countless refugees, then arranging travel from there.
So, I could give a crap about any Russian runners who don't get to go on their little marathon vacation. The BAA's position, by the way, is entirely consistent with all the other worldwide sporting organizations that have banned Russian athletes.