This is true as far as it goes, but any argument like this is a little circular in that we are relying on texts that were transmitted and transformed for hundreds of years before settling into the form that we have them today to justify what to do with those texts...
It's also a little ironic to cite the Mahaparinibbana sutta (DN16), since that's one of the suttas that is likely to have been added to over a long period, so if one subscribes to a textual-analysis approach it's one that would be looked on with suspicion.
One could also cite numerous suttas about choosing a teacher and asking advice from others...
I certainly find validation for what I've learned from various teachers in the suttas that I have read. However, even when working with the fixed Pali canon of suttas there can be disagreements over some doctrinal points, as we see quite often in discussions here. And when it comes to meditation techniques, the suttas are sufficiently general that almost any modern approach that I know of can easily find suttas to back it up.
Which, actually, I think it fine. It indicates to me that the Dhamma is quite robust, and that a variety of interpretations are possible without destroying the essence. Some will resonate more with particular people than others.
Mike