I think you're misinterpreting the use of "high level" here. It's not that the candidate needs "a high level of understanding", it's that they need "a high level understanding". "High" here is in reference to the level of abstraction.

A high level understanding could mean something as simple as understanding the request-acknowledge nature of TCP, as opposed to the 'fire and forget' mechanism of UDP.

As to the claim that virtually no javascript developer needs to know TCP, that becomes less of a truth the closer you get to experimenting with tech like websockets, which is increasingly becoming part of javascript developers' jobs.

I think you're latching onto a narrative that you already made up about the poster before you even began this debate.

There's a large leap between what the poster is saying "I ask a wide range of questions to catch red flags if they get all wrong", and what you're saying "the poster is saying that javascript devs NEED to know TCP! Obviously an elitist filtering mechanism!".