The idea is that if Poland or Bulgaria or Romania give the Ukrainian air force aircraft that they are used to flying they can inflict even more damage on the Russian troops. But it it a balancing act.
These countries won't give up their aircraft unless someone replaces them, hence the F15s.
The delicate part is demonstrating that it is Poland/Romania/Bulgaria providing the aircraft on a bilateral basis and nothing to do with NATO. If it is seen as NATO aircraft attacking Russian troops we are in a whole new ball game.
I think the word delicate is underemphasising it.
(Can something be underemphasised?

)