From my perspective, somebody who has not attended university but still learned everything they need to do a dev job well has demonstrated something really important：the ability/motivation to learn new academic things in their own free time. So many people seem to completely stop learning once they leave university. I work in HFT, where things are constantly changing and people need to be at the top of their game to stay competitive, so the ability and willingness to keep learning new things is vital for success, as the majority of what makes a quant-dev successful will be stuff they learned on their own/on the job, not stuff they learned in uni.

In this sense the biggest value of a degree in hiring is just the crude proxy it is for IQ and conscientiousness. That being said, I've never interviewed anyone without a degree, but then I've never seen a candidate without a degree either. I have hired people whose degree is completely unrelated to the role, because they demonstrated the ability and interest to learn something outside of what they studied at uni.

So perhaps part of the opposition to university is due to the (possibly incorrect) feeling that it encourages people to think of it as "this is the period I do my learning", whereas some alternative to university might better promote people to develop a habit of lifelong learning. I.e. maybe people have been jaded by the experience of interviewing/hiring people with degrees from good universities who just spend all their spare time consuming media.

Which is not to suggest attending university is a bad idea, given its value as a signal, just that it doesn't seem ideal for developing the habit of lifelong learning.