It's a new label but it's not a new concept at all. Factional bickering that weakens a broadly appealing platform has been happening since we invented partisan politics. Just on the left, look at the French Revolution and the socialist splintering in the early 20th century.

It's not a thought terminating cliche either, it's a real and tangible problem. The rise of the internet has made targeting and widening these fractures easy and effective. Any bad actor can trivially propogate a message to any number of people, making them naturally coalesce into opposing echo chambers. This chart may as well track social media use during election years.

It's true that "purity testing" is often used as a bludgeon to stifle criticism of obviously regressive policies, but it's unfair to completely ignore the kernel of truth about the opposite end of the spectrum. When a bloc of voices is lodging criticisms with no constructive platform, there's no value being added and they may as well be opposition.

It's often not that hard to tell when that line is crossed either. If your claim is candidate X is vile but you have no real option Y as a substitute, then your attack can only be strengthing the opposition. There is a political reality you have to operate in; the system of election and popular opinion automatically limit your options. Being vehemently opposed to a subset of policies does not mean supporting that candidate is automatically the wrong political play.

If you're truly a bright-eyed idealist and can't stomach the political sausage making then you'll have to find another approach. If you want to use the system in place (for example, USA's FPTP and electoral college) then you have to put in a ton of work and political maneuvering. If you don't believe in the reformist approach then you have no skin in the game and shouldn't be complaining.