Of the two positions stated, theirs in the interview feedback and yours here, yours is BY FAR the more reasonable. That they would even feel free to say that to you indicates a bizarre level of entitlement or pre-employment loyalty there, one that's made worse if it was the literal truth.

Entitlement is like an iceberg: what you see is just the tip. There's always a whole lot more right behind it.

So let's go there. I haven't seen anyone else bring it up yet, but hypothetically, let's say you drank the koolaid about their brand and, to increase your chances, you did spend a wad of cash (that you probably can't easily afford) on their product before you even got to the interview. You walk in with that experience, able to tell them you've had their lessons and talk about their platform from a user's experience, etc. Great!

Now what? How much farther does that actually get you? Not a goddamn bit, IMO, since you're still behind anyone who has ever worked on a product of their own brand, and/or kissed whatever other invisible and undefined rings they want most but were not actually disclosed in the job posting. You spent all that cash, but your deficits as a candidate are still hanging in the air: you've never actually worked on it, just familiarized yourself with the product, albeit at a cost to you.

I am so glad you are writing this from the perspective of "should I have spent the cash?" rather than the perspective of "I spent this cash and now I'm out" because above and beyond the weirdness of their behavior, the last place you ever want to sink cash is on a job posting that can't be bothered to include its most important requirements. Doesn't matter that it's a well known company, individuals and departments can be unethical too, and these certainly were.

It's also entirely possible there's an internal battle going on over this job, with some insisting it should go to someone already in-house and others, possibly even company policy, forcing it to be posted to external candidates -- but in reality it has already been decided and they are just going through the motions of ticking the boxes until they can hire the one they wanted from the start. If so, you were never going to win it, and the whole thing was a gargantuan waste of time.

Add to that the fact that the posting itself omitted the company's own most important requirement for the job, and I can only add to the chorus of people here who have already said you dodged a bullet.

Relax, you did good. Glad you made the decision you did. Best of luck in your job hunt.