Without Javascript, the tab can be a label that activates a radio button next to a hidden div that also has a input:checked~div { display: block;} CSS on it. No Javascript required.
In this case the search is instant. Without JS you would have to have a submit button and wait for the request. Even if you also added a button the JS version it would still feel more responsive, as again, you're not waiting for the request.
[1] https://www.w3schools.com/howto/tryit.asp?filename=tryhow_js...
Additionally, when it comes to client-side spreadsheets I have seen far more terrible half client-side, half server-side implementations (being only able to sort within a page, instead of across all pages of results). If I had to choose one, I'd choose a world were all we had were server side spreadsheets.
I have also seen the horrible half/half implementations you mention where it should have just been implemented on the server side, and I totally agree with you there.
However it was just an example to show that a unsubstantiated blanket statement like “responsiveness never” is just wrong. I’m not saying doing search in JS is always (or even often) better, but it can be sometimes if done well.