Your assumptions are false.
It is actually more profitable for me if I say that these algorithm tests are BS and completely study-able, and even the worst engineer can pass them. That easily translates into "here! buy my book and even YOU can get a job at Google!"
It's actually a much harder sell when, really, it's more like studying helps someone perform better, but only to an extent.
Oh, and the acquirers encourage studying and prep, just as most of the top tech companies do for their normal dev (and non-dev) candidates.
To be clear: my experience is that the algorithm interviews, when done properly, work reasonably well to identify intelligence/problem solving and coding skills. Those are more important at some places than others. They can work well for certain places -- but not at all. I do not think that all companies should use this process. It's not right for all places, but it's reasonably good for some.
As for Google being proof of the value of algorithm test: I think you're pulling that from a twitter conversation very much out of context.
Context: Some people were arguing that the algorithm interviews offer zero value - that you'd be at least as well off, if not better off, hiring at random.
If they truly believe that (and they appeared to be sticking to this argument), then it's absolutely relevant that not just Google, but basically all the top tech companies, use this hiring process. Could a randomly selected group of engineers build the technology at companies like Google, Facebook, Amazon, etc? I suppose that's what they must believe, and I find that pretty absurd.
If they had argued that these interviews sort of work, but aren't nearly as good as some other interview processes, then you're right that the company about Google and other companies wouldn't be relevant. And I also wouldn't have brought it up.