That sounds like a "no true scotsman" argument. Even passive investors need to pick some methodology of how to pick assets and how to relatively weigh them, and while you can make that as mathematically simple as possible, it's arguably an active decision.

Or would you say that e.g. an ETF tracking MSCI ex-US is not a passive fund?

I’d also argue that "passive investor" applies more to the buy and hold strategy when paired with low engagement in the account (few transactions, or scheduled transactions).

I’d consider someone that puts $50 into Coca Cola stock every paycheck a passive investor