Hello!
The option is blocked for the homepage because a unique setting exists to affect the homepage title. WordPress’s homepage configuration is a mess: you can turn it into a blog, change the page IDs, delete it, etc., so we had to make some questionable design decisions.
If you go to “WP Admin > SEO (left menu) > Homepage Settings > Additions,” you can change or remove the floating part. This is precisely what I posted in the other support topics, so I’m a bit confused about your question.

The homepage’s on-page SEO settings should also tell that you can overwrite the settings elsewhere. Perhaps I need to word this better.

Still, I recommend using the plain brand name as the main title and putting whatever else you need to tell about the site in the additions. Google is prone to ignore your title otherwise.