Except that they already said "no, installing an OS doesn't void your warranty, that was just a mistake". But the people actually processing RMAs are clearly not aware of this. That's the problem. I can not trust the company to process an RMA, because they have demonstrated that they do not train their staff to do that job competently.
Warranty, Return Policy and One-Time-Exception are three different things.
Since the previous laptop had issues, and a shit-storm was kicked off, they said it was their mistake and that they will accept the return, even though it was being returned in a non-original state (one-time-exception).
This laptop apparently has no issues that they can detect, and was returned in a non-original state. Hence the RMA is not valid.