The pentagram icon is the original symbol for the fediverse, created back in 2018 by Dr. Quadragon and Eukombos. It’s a great depiction of the decentralised nature of the fediverse, and has been serving the community well. However, its design is a little too complex to be used at small sizes, as you would in text or in a button. It’s also only available in image form, not as a typographical character.
I think they have a valid point. Currently on my website I use a Mastodon logo next to email and git and all that jazz. It's not ideal, as it's not so important that I'm on Mastodon specifically (and I might move to a self-hosted #Seppo instance in the future), but the existing fediverse icon would not work well at that scale.
It's a huge branding effort to make it catch on though. And part of me likes the pentagram better.