The sell is that my manager can send an Excel spreadsheet to everyone and everyone can open the spreadsheet and edit it at the same times while seeing everyone else do their edits. What's the non-MS non-Google solution to this?
Anyone can edit it and it also might get randomly corrupted. It’s crap, especially if some people are on Macs.
Open-Xchange supports collaborative editing of spreadsheets. Mailbox.org uses that for their email service, and you get access to their online office suite when you subscribe. I can't speak to the quality of the shared editing, but their online office suite is fine for basic stuff.
Zoho.
I haven't used any but there are several it seems: https://european-alternatives.eu/alternative-to/google-docs
NextCloud looks ok.
For some reason I thought it was open to the public, but France also maintains a full sovereign cloud office suite for use by civil servants: https://lasuite.numerique.gouv.fr/en
Maybe one day they'll open it up publicly.