"same functionality" includes a LOT more than just the basic functions. It also includes voltage tolerance, noise resistance, and all of the other things you dismissed.

But still as the author explains "If you're not reaching for the limits (like max VCC or precision) and have some luck - that might not cause issues." And I didn't dismiss that...I said "just simply don't meet the voltage tolerance and noise resistance levels of higher grade designs".

I guess what triggered me to write my first comment was the fact someone buying a "TI" something chip from eBay when it lists "China" as the seller's location and with a seller's name other than something easily verified to be TI, then it seems there is no pretense that the thing being sold is genuine TI. But the word "fake" holds the connotation that there is some deception going on. So that is why I don't think fake is the most appropriate word. I just hear that word used too much to disparage chinese made derivative imitation products. Someone who knows they don't need the noise levels and voltages tolerances can be perfectly happy and even benefit by using these instead of the more expensive genuine TI.

555s are chips that sort of fundamentally sit of the boundary between analog and digital. Their analog properties matter in most cases.

So, are you saying that if I suspect that I am being defrauded, it's not fraud anymore?

Not suspect. But know.

So, how do I know (and not just suspect) that a particular seller is not selling TI parts?

Well there is no way to verify that the device was manufactured by TI. That itself doesn't let you know as a matter of truth, but that does let you as a smart shopper know that it is likely fake. You can make an inference since (A) you are buying on a website that is known for providing a means for counterfeit goods from China, and (B) the seller is located in China and has no info that reliably identifies it as a seller of genuine parts.

things like tolerances and noise figures _are_ part of the functionality.

That may be a valid point for a TI-specific part. But if the buyer is just wanting a generic 555 Timer, those aren't part of the specs. The 555 is effectively a black box (https://en.wikipedia.org/wiki/555_timer_IC). Although again I do realize now that the author was primarily attacking the use of "Ti" here.

No, your parent poster is right. Noise-tolerance, induction-safety, aka as working in every case as specified in the data-sheet - are part of the core-product. That is not just some luxery quality issue, this is core functionality. Without it, the chip may fail when placed near some EM-Sources onboard or go haywire on hot days. There is more to Layouting, then connecting A to B.