In a situation Steadfast Armasaur is destroyed with its ability on the stack the Release notes, and the gatherer rulings by extension, says it still deal damage, using last known information.

Have there been any rules updates after Triton Tactics have been printed that would make this correct, or is this a case where the text in the Release notes is “just” a LKI reminder text and checking for a legal target was overlooked?

It states “creature blocking it” which, in my understanding, indicates it needs to be actively blocking it, not falling under “having been declared as a blocker for it”.