They are missing something major here and getting bogged down in some technicalities. Open source has no alternative to big tech because big tech commoditised stuff that's useful whereas open source commoditised stuff that is interesting to the developers.
When I sit down at my mac, I have a working and very polished calendar, mail client, todo list, contacts, note taking app, music player, browser, photo editing and library management tools, video call and conferencing software etc. And all of it syncs with my phone and my tablet out of the box.
When I sit down at a Linux machine, I have a calendar that breaks every 5 minutes and I can't share anything with anyone without futzing with iCal feeds and hiring another provider, a mail client that is ugly as sin and doesn't integrate with the calendaring or contact management stuff at all, a job and a half to find a note taking app that actually works properly, a todo list app that syncs with nothing, a spreadsheet package that crashes whenever I try and print something and oh hell I give up by then. And the answer to this? Roll out nextcloud on a VPS. Kill me, with a spoon. This is not freedom, it's just slavery of another kind.
I just want to get shit done. Big tech covers that. Please take this as a recommendation to tidy up all this hell and just help people to get shit done and then it will be an alternative to big tech.