As far as I'm aware, no one ever said the 'key feature overhaul' is tied to Update 15.
Frontier pretty much have, in that Update 15 is the only time it could be released and still fit the original schedule, and the more recent descriptions of U15 contents are compatible with something they might call a "major feature overhaul". They didn't use the exact same phrasing, but given they called it two different things in the original May post it doesn't seem to be something they're attempting to build a marketing brand around anyway.
From the original post in May: "Work has also begun on a major feature overhaul for 2023" and
"Early 2023
Key feature overhaul"
From their U14/Live/Legacy announcement: "Aftermath will see its next major release with galaxy-wide implications in Update 14, with more to come in Updates 15 and 16. What we can say for now about Updates 15 and 16 is that they will both contain new content and gameplay mechanics linked to the unfolding events in the galaxy."
From 9 January Discovery Scanner: "We're also happy to confirm that we are on track to deliver further updates this year, targeting Update 15 in April and Update 16 to follow later in the year."
It doesn't technically rule out U15 being unrelated to the Key Feature Overhaul, and there being a separate and un-numbered update planned either before U15 or between U15 and the end of June to deliver that (possible, if it's something which could be done entirely server-side), but that certainly doesn't seem to be a natural interpretation of those statements.
(I agree with you that it's therefore almost certainly "more Thargoid War" of one sort or another)
Oh? What makes you think so?
I've seen bugs and non-bug issues mentioned only on the tenth page of an obscure forum thread get fixed, player actions that no-one was particularly trying to draw Frontier's attention to get brief mentions in Galnet, and so on. I doubt that they read absolutely everything - it's a big forum with enough activity that just reading all of them would be a full-time job, and then there's Reddit and other discussion places too that they need to cover - but they certainly read a surprisingly large amount of it.