Yeah, I figured that was likely the answer. Thanks anyway, adzenith.
In an attempt to hopefully prevent myself from executing future stupid operations like this (without hope of recovery, that is), I've installed the following plug-in:
Won't stop me from making the occasional dumb undo-redo mistake, but at least if I get in the habit of Ctrl/Cmd-S saving a lot, it may allow recovery. Though now that I think about it: my user.settings has 'save_on_focus_lost' set to true, so I may just get history without needing to Ctrl/Cmd-S--I'll have to experiment with local history to see if it saves history on focus lost too.
PS Update: I upvoted the following on UserVoice too:
sublimetext.userecho.com/topic/2 ... o-history/
sublimetext.userecho.com/topic/1 ... all-files/
sublimetext.userecho.com/topic/9 ... selection/