Seems like there is a better way to use Spell Absorbing Life against the shroom. Don't activate your trap, instead wait for the Sylvan player to flip the shroom up, THEN chain it with Spell Absorbing Life. Supposedly that will cause the shroom to miss its activation timing.
Wherever you heard that from was mistaken. They choose whether or not to activate Komushroomo's effect before you can even activate Spell Absorbing Life, and an already activated effect can't miss its timing since missing the timing is only a mechanic that prevents something from being activated.
I think it has something to do with chain link too. If spell absorbing life is CL2 and above I'm pretty sure Kumo will miss timing correct me if I am wrong here though. Because I believe the "when" effect does not go off effect does not activate after the flipping. It has to resolve right after it was flipped.