Those files are ES modules served (at least to my Firefox) over HTTP/3 with unique filenames and a max-age=604800 cache header, so I don't think that's any less efficient than if they were served as a single bundle.
This means that if any of those individual files are updated in the future just that file will need to be re-fetched, with all of the others staying served from cache. A single bundle would need to be fetched in its entirety on any change.
Activitypub which, mastodon is based on, requires active cryptographic signature of everything. It's why there will probably never be a truly minimalistic mastodon client and they'll all be bloated javascript (or otherwise) applications and not minimalistic HTML elements doing POSTs/etc. This choice has made it irreducibly complex and heavy.
Yes, this is the type of blinders on thinking that activitypub users sometimes have. Take them off and look at a real minimalistic protocol like webmention https://www.w3.org/TR/webmention/https://indieweb.org/Webmention . It is possible to send and receive without any complex application at all.
Also, it's weird that you say I'm "completely wrong" given the uncontroversial single claim I made about the cryptographic signing being what makes it complex. Since you wrote a client you have to know this is true even if you disagree with the 'complex' bit. Unless you just used a module that did it for you? https://imgs.xkcd.com/comics/python.png
It does look pretty though.