They seem to have a questionable interpretation of the AGPL isitreallyfoss.com/projects/onlyoffice/

Considering the specific clause they cite allows

Requiring preservation of specified reasonable legal notices or author attributions in that material or in the Appropriate Legal Notices displayed by works containing it

A logo isn’t attribution, and even if it was there’s no way it could be considered reasonable to require it if they don’t allow you to use it.