I don't get why if they don't mean unlimited just say up to 20TB/mo.

Possibly because there actually wasn't any limit. Maybe if a handful people were exceeding $LOTS TB, they don't care, but if 60% of users exceed $LOTS TB, the service becomes unsustainable. In this case, the service really is unlimited (there genuinely is no limit that you're not allowed to go over), and if you wanted that effect, advertising a limit would be net negative — a high limit would encourage the "too many users use a lot" case and lead to the same result we get now where the plan has to be canceled for unsustainability, and a low limit would defeat the purpose.

Because it isn't linux iso's.

were they being serious, I can't tell.