I stated how Nikon and Canon traditionally defined it. Mentioned when they both, for technical limitations, changed their definitions and how Nikon went back to the traditional definition used by Nikon and Canon, how Canon didn't and how Sony joined in that traditional definition.
That's not my personal set of criteria. It's the history of the term. That you lack the knowledge of history doesn't make it any less true.
Intrigued, I looked at old press releases for the 1D and 1Ds (e.g.
https://lens-db.com/camera/canon-eos-1d-2001/,
https://lens-db.com/camera/canon-eos-1ds-2002/). BTW that's the only other site besides DPReview that still has the old press releases, and multiple ones at that (dpreview only has one, but includes the full headers and dates).
When introduced, the 1D was the flagship no question about it. It's not stated as clearly what the 1Ds was to Canon at the time of release, but that it "tops the flagship range". Later iterations are more murky about which one is the true flagship, probably because they released the Mk versions alternating each year. It gets bit silly with the 1D Mk IV where they sell the APS-H crop as a feature, but I digress.
So it seems your recollection here is certainly correct. Anyways it's always clearly stated in these that they are 1-series professional cameras, and many features beside resolution are touted that make it a 1-series.
However, at present - especially looking at the direct quotes from Canon in this thread's article which I hope you've read - it is 100% clear that
Canon considers the R1 to be the one and only flagship camera they have right now. It's also the only 1-series professional camera right now.
The R3 was a different beast, from the unusual 3-series numbering to the fact that Canon themselves had to clarify multiple times that it was not the flagship camera
to them. That they've relegated the high-MP alternative to the 5-series now makes this very clear, there has never been a 5-series that was considered by Canon to be their flagship.