This is interesting given that I heard on the grapevine that some major cloud players are actually using AWS on the back-end even though they are advertising say as.. "GCP".. wonder if anyone can confirm or deny...
By nature large companies have massive global teams and there is no single provider for anything. Team A could using AWS, while team B cloud be using GCP, and team C is using Azure. Just because team B says they are using GCP doesn't mean the others are lying. Or, that there is anything weird going on.
Actually I meant to say that the news on the grapevine is that the big cloud players (GCP etc.) are potentially outsourcing demand for cloud services in excess of their capacity to AWS...
To be blunt. No. This is totally absurd and would be trivial to detect if true. On the legal side, you'd be breaking all sorts of ToS, Security, and Privacy agreements (you'd have to disclose this via a data processor clause). On the technical side, latency would also be so obvious to detect this too. They are totally different hardware/software platforms and would have different characteristics (as proven by this thread). On the business side, no AWS/GCP/Azure CEO will ever do this, staff would totally be aware too. This is 100% bogus.
I actually did not even want to reply and this make zero sense but felt invested. Whoever told you this doesn't know what they are talking about.
That said, hosting providers who historically ran services on only their own hardware could certainly be load-balancing to cloud hosting providers. This is certainly not what the comment you responded to implies. However, it is something I'd expect people to get confused about..
You are right, a good example might be Heroku. Heroku is a hosting platform that is hosted on AWS (maybe parts or all; I'm not sure).
"Heroku’s physical infrastructure is hosted and managed within Amazon’s secure data centers and utilize the Amazon Web Service (AWS) technology. Amazon continually manages risk and undergoes recurring assessments to ensure compliance with industry standards." See https://www.heroku.com/policy/security.
This is absolutely not happening. All the major clouds are investing billions into infrastructure build-out and I've toured their data centers first-hand.
I would question the people who told you this rumor as they either have no clue how things work or are blatantly lying to you.