@blogdiva can't say anything about the other CMS, but...
" #BackdropCMS should do" sounds a bit odd to me. If you want a feature, create it.
Backdrop is an #OpenSource web CMS. No platform or company, it's software built and maintained by a community. So, Backdrop - as software - doesn't "do"... 😆
No ActivityPub implementation, but a module to publish to Mastondon, hence the Fediverse, by writing blog posts exists: https://backdropcms.org/project/mastotoot 😉