GP is talking about intermediary proxies, CDNs etc. that might be unhappy about long-running connections with responses trickling in bit by bit, not doubting that it works on the client side.

That said, I'd be surprised if proxy software or services like Cloudflare didn't have logic to automatically opt out of "CDN mode" and switch to something more transparent when they see "text/event-stream". It's not that uncommon, all things considered.