Did the process changed since it was first published in 2022?
Also, at the end of that post there are two good questions and it will be helpful, if there was a response to them.
1. When you say to install experience builder, does that need to be on the machine that hosts the enterprise?
2. Is it possible to add custom widgets to ArcGIS Online too?
I'm not sure I know the procedure for #2.
But from what I understand, working on something similar right now for #1.
The enterprise (portal) has an option to create an experience builder widget. It takes a URL path,, because it is a URL path, in theory it could be on any server, or even in a CDN potentially. The only caveat may be, if it is under a different domain, you might need to adjust CORS settings.
note, The permissions to create widgets and update those settings are not available in all user licenses. So if it is missing when you try to add, that's probably what is going on.