Yes, it's a problem when it's to the point of completely disassociating from you given the opportunity to do so, and the comment was made outside of work, in a completely different forum, and not to them or about them. They're not willing to engage with you and try to change your mind, or even just to continue working with you (without being forced). They're jumping straight to outright ostracism, or as close to it as they can manage. And anyone who doesn't do the same is next in line—guilt by association is a big part of cancel culture. Often it's not about what you did but rather about your failure to publicly condemn and "cancel" someone else for what they did (or perhaps only have been accused of doing).

I'm not saying they should just ignore whatever specific thing it was that gave offense, though sometimes that is the right approach in a professional context. Choosing to working together while ignoring irreconcilable differences is sub-optimal but better than not associating at all. That, however, is something that people should choose for themselves, not something they should be forced into.

> Implicit in this is the claim that people don't already do this.

I'm sure some people do, and if they don't feel that they can continue to associate with someone based on what they've personally done then I support their choice. In the vast majority of cases, however, a more measured response is warranted which doesn't involve burning all bridges and driving the offending party into the outer fringes of society where they are likely to encounter others who were similarly exiled and become ever more entrenched in their positions.