To be honest, I am not sure how it could be thought the updates were cumulative; if they were then each update would have to be bigger than the last and they clearly aren't.
TBH I would happily be proved wrong with the majority of games as the thought of installing
25+GB of updates for Super Smash Bros fills me with dread

But, not trying to be rude about it but the only post on this thread saying they are cumulative is you and there other forums around stating (by many users) that they aren't.
Obviously it's no biggie to download just the latest update and see if it works [ and then hope they don't have to be installed in order

] but I am just saying that, from what I've read, it's very confusing.
EDIT: LOL - well there goes my theory, even with all 10 updates installed, Overcooked 2 still says there is an update to download.