After experimenting a little bit, it seems to work in the way you described, which sucks. Quickening's "+X Combo count chance" and True Punishment's "+X Additional Combo Count Chance" does help me increase combo by a bit more whenever I do gain it, but the rate at which I gain combo by hitting enemies feels the same as without them. I guess this is a bug? or at the very least extremely inconsistent wording and mechanics.
The only "fix" I can think of is to clarify and standardize the wordings, but I really do hope they change the mechanics and standardize them, cause this neg can't be offset by any mods unlike a lot of other stats and on higher dispo rivens makes the combo counter completely unusable and thus making the riven basically unusable/unwanted, which wouldnt be fair when comparing that neg stat in particular with other neg stats that you can actually do something about.