Opensource

Attribution isn't the problem. They try to circumvent the AGPL by making their trademarked logo part of the attribution, thus basically forbidding any of the rights people normally have under the AGPL.

idea of requiring attribution is specifically called out in Section 7, item B.

But requiring that the logo be retained is not.

Now, it's not entirely clear what they even are asking for. If the ask is that the OnlyOffice logo be included in a 'credits' page (which is perhaps a reasonable interpretation of "you must retain the original Product logo when distributing the program") then it is much less problematic, although perhaps still beyond requiring attribution, than if they are trying to demand that, say, the favicon of the webapp must not be changed (especially if their intent is also to say that you can't distribute the program if you don't change the favicon because it would be a trademark violation).

Now of course, if they have written all of OnlyOffice in house, and not had any external contributors or used any external copyleft code, then they can re-license OnlyOffice on whatever terms. If they were bound to the licence by including other people's AGPL contributions, then they have to follow the licence themselves, and cannot add arbitrary additional restrictions.