Re:
That is definitely one well stuck BB cup, I think galvanic corrosion between the alloy cup and the steel BB shell must be the cause, as commonly occurs with alloy seatposts in steel frames, plus not using an anti-seize compound on assembly and leaving it untouched for donkey's years. Might be worth pouring hot water over it a few times to see if you can break the bond through thermal shock, though slightly less likely to work when alloy is inside steel than vice versa, then again filling with a penetrating fluid and leaving overnight.
Alternatively freezing the alloy cup and heating the steel BB shell could also help and if you could get a large socket on the tool, plus breaker bar with extension on it like a scaffold pole you will get more leverage without chewing the tool to bits.
Caustic soda will work but is very nasty to use and will take several goes to work, plus you have to seal up the frame enough to contain an effective amount inside it.