Undermining is a proactive action to weaken something. Refusing to invest may or may not be undermining depending on prior state and intent (a lack of action with no history can still be proactive, it simply lacks prior state information to assess). Withdraw of investment with clear negative implications is undermining, which is what happened here.
You can weaken through direct attack, withdraw of assistance, or you can even strategically offer assistance to weaken (identify largely unknown harmful processes and help them grow), amongst an array of other strategies, direct and indirect.
The grey area is if you've been a bystander to action and refuse to act one way or another because intent is the only information you have and you can't (fairly) gauge on that alone. This isn't one of those cases, this is a clear case of withdraw of active assistence which undermined CentOS. Business processes often work in reverse of what is explained. It's not likely Red Hat made a decision and then informed CentOS there would be consequences. It's far far more likely Red Hat looked for decisions that had the consequence they desired, chose one, and then informed CentOS that wasn't their intent.
These sorts of games of misdirection of intent and responsibility are played by businesses and sociopaths all the time. Decisions like this are highly strategic and implications are discussed throughly, meanwhile the narrative is crafted to then look benign and non-strategic.