Daily chess puzzle: Check Mate #467(2)

Where is the cook in yesterday’s problem?

Seirawan v Barbero, Skien 1979
Yesterday, I gave the solution to this White to play and win problem, but said that I had a shock when writing up the posting that my engine immediately flashed “0.0”.
The reason becomes apparent only once you see it!- as so often in chess, moves or positions become easy to understand in retrospect.

Purdy’s writing is yet again apposite:

Imagine the threat could not possibly be executed. Then what would be by best move? Try out each attractive move…visualise the whole position…after this move of yours, and then work out whether the opponent would gain by executing his ‘threat’.

In line a, after 4…Nf7 5 Be5, Black doesn’t need to move his Queen, but can based on the Purdy principle of ignoring threats, or simply as a desperado, or to try to give his King some luft, can play 5…Ba6+

Then, perhaps amazingly, there is nothing more than a perpetual check. 6 d3 Bd3+ 7 Kd3 Qf2

If White plays, as planned, 8 Bc7, then 8…Qf1+.

White’s best is then to accede to a perpetual, say by 9 Kd2 Qf2+ 10 Kd3 Qf1+ 1/2-1/2. If instead
9 Kd4?? Qa1+ and the king and queen are skewered;

9 Kc3 Qa1+ same;

9 Kc2 Qc4+ and forks the king and LPDO Bc7;

9 Kd4 same.
I wonder if Barbero merely resigned after 5 Be5? The game score isn’t in Megabase of chessgames.com.
I wonder if the players knew?

Chess is indeed a deep game.