Are you considering preferential with proportional? Preferential means you order your preferences. No more, no less. A proportional system (of which STV is a quasi-member, being somewhat proportional, but not quite as purely proportional as most other proportional systems) is one where people's overall wishes are better taken into account, and the resulting parliament is a better representation of the will of voters.

Instant-runoff is just a way of having a bunch of first past the post elections in one go

Eh, sort of. It's a way of doing a runoff voting system all in one go. It's like the system parliamentary parties use to choose their leader, where they do one vote, eliminate the last place candidate, and then do another vote, and repeat. It's not really FPTP because the highest-scoring candidate in each round isn't what's relevant, the lowest-scoring is.

But I think we both understand how the elections work from a mechanical standpoint. You're just not clear on the correct terminology to refer to them.