I figured someone here might be able to explain this to me better than the ISP (or tell me they're wrong).
A lot of domain registrars provide DNS hosting, but often only after the domain has transferred, and only after the DNS is set to use their servers. This means there is a delay between (a) switching to the new DNS and (b) adding the host records to it, resulting in potential downtime for anything that relies on the DNS.
My question: If the time between (a) and (b) can be reduced to almost zero (such as by making both changes in a single hit via an API), I assumed that there'd (almost certainly) never be an incorrect DNS response given and therefore no propagation delays should apply. However the company in question told me that there would still be a propagation delay but disappeared from the chat when I asked where.
Am I missing something, and if so what?