You say "tax money", but this project isn't a government project or using public money at all. As for contributing back to Nextcloud: there is a long list of Nextcloud partners [1] that contractually obligated themselves to contribute back to Nextcloud for every user they onboard. The company in this article has not.

[1] https://nextcloud.com/partners/

Oh what! Wow, that' misleading, although I guess it doesn't explicitly say "public" or "government" everywhere. Hm.