Hmm just realized Geany defaults to Firefox, Ubuntu must have overridden it (and Mint is using the Ubuntu package).
For a Geany built from git on LM I get "sensible-browser" in the preference. So its probably not Ubuntu, somehow Geany build selects that value, but I ran out of time to find how/where.
But help does work for me (opening in the preferred browser, chrome, selected in the LM desktop "Preferred applications" dialog).