The legacy buff, when it was first introduce, only applied to raids/content 2+ expansions old. Which is why it would only apply when you were 11+ levels higher than it, which is only possible on content older than just the previous expansion. The immediately previous expansion has never been their priority when it comes to players soloing "older" content.
That seems to have changed with this squish, but this would be the first time the immediately previous expansion raids were entirely soloable.
EDIT: Clarify legacy buff when it was initially implemented, vs now.