Hmmm, works the same for me, inserts the selected completion (current svn, new config, tab keybinding)?
Same setup here, except not new config and the "Complete word" binding it's at the default of <Control>space. If the autocomplete list is showing, it cancels it, if it's not showing, it shows it. A few snips from the manual:
Ahh, I'm talking about the "word part completion", default binding tab and which inserts the completion (or as much as is unambiguous). you are talking about the misnamed "word completion" which calls the better named editor_start_auto_complete() to show or swap the menu, as it says in your quotes.
There isn't a "accept the selected autocompletion" that accepts the whole completion, Geany relies on the return going to Scintilla.
Cheers Lex