> it is a regulatory requirement to disclose the fact that you are collecting data
Not if the data isn't PII, no. Not in any way shape or form.
> The few exceptions that exist are only applicable in cases where there is no potentially identifiable data collected at all
The whole point of collecting "anonymous usage data" (which is what telemetry usually does) is that it shouldn't be possible to attribute to a physical person, and thus not be PII. As an extreme example, you could take the most typical form of telemetry: a feature usage count. When a feature is used, the telemetry collects a (+1) for that feature. The only long term stored data is the total count N for each feature across the entire user base. Of course there is no PII stored.
> which is obviously not the case here.
Why do you say that it's "obviously" not the case, when there is no indication about what data it is, other than the Dropbox representative saying precisely that there is no PII collected so the GDPR isn't relevant? There may be PII (in which case they are both at fault for not disclosing, and complete asshats for lying in the support forum). But it would be a pretty uninteresting discussion once one assumes that...
Obligatory "I'm not a lawyer either but I've implemented telemetry in software and had those implementations thuroughly analyzed by lawyers a couple of times"