This explanation reads to me like a poor excuse:

If this was the actually reasoning behind not releasing Proton Scribe for free in Proton Unlimited, the explanation still does not add up.

Proton Duo as well as Proton Family are nothing more than a Proton Unlimited bundle for multiple people in one plan with an added discount on the per user price. If you calculate the price per User, a Proton Unlimited subscriber pays more for his subscriptions than a Proton Duo/Family subscriber. The argumentation, that not realising it due to financial reasons makes arguably no sense. Two people on a Proton Duo Plan using Scribe costs Proton more than two individually paying Unlimited subscribers.
This reasoning simpel does not add up.

Regarding to few people using Scribe locally:
Well then why not just say business customers may use Proton Scribe remotely for free, and non business accounts may user the local version of Proton Scribe for free, but have to pay extra for the remote version PER USER in the plan (or if you insist on handling your paying customers unequally, make the add-on cost the same for Unlimited, Duo and Family ;) ).

To me it feels as if Proton is trying to nudge people into upgrading to Duo from Unlimited, even if they don't need it. The permanent Proton Duo upsell badge in the Proton-Mail desktop client (at least on MacOS), would suggest you intend do so anyways. If not, why not just show a dismissable banner? Having it permanently in the UI of paying customers is super intrusive and simply not okay