As you righfully noticed, some parts of what's done on the backend (e.g. with PHP) constitutes the UI delivered to the browser, so these parts are relevant to frontend jobs. Some of that UI requires building network APIs, so that's relevant. UI puts requirements onto the application state persisted somewhere on the servers, so general API design and database structure is relevant, too.
Given the complexity of the modern UIs the browser-side is no longer enough.
Titles are hard. Some companies say "UI Engineer" which is closer to what the modern frontend has become, i.e. full-stack implementation of UI features. The client-server architecture is an implementation detail of a web UI so it's silly to split responsibilities based on technologies used. Technologies are chosen to implement products, not vice versa.
So I do agree with you, in some contexts some backend languages are very relevant.
Personally I started learning PHP because I had to build forms to send emails and later I needed to make changes to Wordpress templates.
But this creates a problem as I actually don’t understand how PHP works at its core. I would fail interview questions and I have no desire to deep dive into the language. I might be able to do the job but since I don’t understand a fundamental programming concept I might be deemed not qualified for the role.
Also I like UI engineer as a title!
Tech deep dives are off the table for me. I only do conversational style interviews. I try to feel them out as much as possible but if I get in an interview and it slips into deep-dive I just say "I don't know" to everything to end it ASAP.
Tech deep dives are a really bad sign. At the least you are being treated as a commodity.
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