Highway Blossoms 1.2.2, 1.2.1, and 1.2 have been released now, and as such, the instructions written below this comment no longer apply to these versions — the version numbers are now in full alignment, and the new 00-README.html file has the correct set of instructions for applying the patch.
A big thank you to @freesw and @Fortex8 for helping out in the comments!
(※ Apologies for the lack of official response — for some reason, it seems that itch.io doesn’t send notifications for comments here…!?)