GEQO's only function, basically, is to search for the join order to use. If you're constraining the join order completely with join_collapse_limit=1 then forcing the GEQO path to be taken would just add pointless overhead. (If it does anything at all ... I don't remember the logic exactly but we might be bright enough not to bother with GEQO in such a situation, regardless of geqo_threshold.)