Correct, the pic shows what happens after 1.Kb8 and after 1.Ka8 the White bishop needs to go to b7 instead of a6, so that the Black knight on b6 stays pinned so he can´t take the mating queen.
Well done.
BTW I was lucky enough and found the solution in a blitz game 😃
https://lichess.org/ASOuUAucz3FG