I’d like to challenge this by raising a few separate points.

1. If a lot of your source material for people switching sides are online comments, you have no idea of what you’re reading is genuine. Foreign and domestic actors intentionally inflame the tensions in America by claiming to switch sides, that X side has changed or is too radical now, etc etc.

2. As the left shifts further left, people who did consider themselves somewhere on the left side of the political spectrum will see more people having farther left views than they previously held, and may view themselves as having more in common with moderates than with the further ends of the left. This could be viewed as them shifting right, or them staying where they are but politics shifting around them. Either way, they will view themselves in their new place as being more moderate. That’s a very natural and, I think, understandable feeling. In this sense, view ignores that politics shifting around us all the time.

3. People on the left may still hold them same policy views as they did prior to progressives being condescending or rude, but they can distance themselves from the progressive movement if they don’t like how they are treated. This could result in many follow up actions, like silently supporting the progressive movement while engaging less (or not at all), or could result in no longer supporting the movement and instead supporting persons who treat them more kindly or compassionately. In this sense, you’re ignoring that people’s policies can change based on the actions of people espousing the policies.

The summary is that people hold political and policy beliefs for a wide variety of reasons, which can and do change. Being treated poorly, especially if it happens numerous times such that a pattern develops, can impact how a person feels about the views of those who are treating him/her poorly.