According to current groupthink, perhaps.  After all the years of people claiming this, I'm still not convinced.  Surely by definition, it's not an altcoin until it's on a separate blockchain?  If no fork has occurred, it's simply an alternative client which follows the current rules but proposes a potential change to consensus rules if enough people run the code.

People might not like the proposed changes in the client, but it's still disingenuous to call them altcoins prior to any fork.  If or when a minority fork takes place, then you can call it an altcoin.