Legitimate interest definition is pretty narrowly scoped. Most of the sneakiness relies on nobody checking or enforcing it.

You are confusing GDPR and ePD. Legitimate interest is GDPR legal basis for processing personal data (it has it's own requirements). ePD triggers when you read/store e.g. browser data and make it available via network. Consent is the primary ePD legal basis and the exception related to that is much narrower (strictly necessary for ISS provider to provide service requested by user).

Generally speaking JS code that makes request to server is covered by ePD while actually processing that request server side is covered by GDPR (there are some nuances to this).