As I said in T332897#8721648 we could claim this is consistent with desktop behavior (which also shows "add topic" there), but the big popup button does make it land differently. In that ticket we restricted it to only action=view, but probably weren't thinking at the time about the strange state of affairs by which a diff page is also view.

Maybe it shouldn’t appear in diff-only views (diffonly=1 URL parameter or user preference set to diff-only), but should otherwise (diffonly=0 or default user preference)? This would be consistent with how __NEWSECTIONLINK__ works on desktop: https://en.wikipedia.org/w/index.php?title=Wikipedia%3AVillage_pump_(miscellaneous)&diff=1204026281&oldid=1203841008&diffonly=1 doesn’t show it, https://en.wikipedia.org/w/index.php?title=Wikipedia%3AVillage_pump_(miscellaneous)&diff=1204026281&oldid=1203841008&diffonly=0 does.