My employer is one of those enterprises; we host our own Exchange server on our own hardware on our own IP address.

I suspect that is not super relevant to this conversation, because there are a lot of signals that Google can use to see that we’re a big fish, and therefore automatically tread more lightly on the SMTP bouncing. We’re running Exchange, for one thing, which is not cheap or easy to use. We’re in the IP address of a big enterprise ISP. We have other enterprise services running on adjacent IPs, like OWA and websites.

Small personal email servers have none of those sort of “ambient” signals; they’re probably running open source email server software on a single IP coming from a consumer ISP or general data center IP space.

So the question might be why Google seems to react more to those ambient signals than other email providers. Because we have no issue at all getting personal emails into a Gmail inbox.

It doesn't feel like "... you need a GMail or an Outlook address" is much of an improvement.

This is unfortunately technically untrue, as Google has beaten Outlook and Yahoo and Mail.ru into submission and into supporting it. It's still a huge security risk, and an inherently proprietary email spec Google designed and implemented without community input and deployed despite significant community resistance.

But technically, it works outside of Gmail.