From what the comment tells, the interview question said that the maximum IS 6TiB. There was no further information given, so I assume it didn't make any assumptions about how it might change.

Even if it would say "it will stay at 6TiB" I would probably prefer a senior candidate to briefly question it, such as "It is surprising that we know it will stay at 6TiB and if this were a real project I'd try to at least sanitycheck that this requirement is really correct, but for now I'll assume this is a given..."

At least if, as the interviewer, I told them to treat the challenge similar to a real request/project and not to not-question the given numbers etc.