Yes, based on the theory, quantum computers are a threat to crypto any cryptographic system as you say. But in reality and so far, there has been no real threat from QC and damage to the crypto industry. Everything is still theoretical so I agree with the view that QC cannot prevent or harm bitcoin in particular and the crypto industry in general.
Quantum computers have not been fully developed to the extent of posing a threat to cryptographic systems, but that doesn't mean it won't in the future. The development of quantum computers will continue and in the future it will surely be a threat, so i would not call it a rumor or anything like that. If it were a rumor, BTC developers would not be thinking of developing quantum resistant algorithm that would be resistant to the threat of QC's.