Another article, much better and presents in more detail that Olvid was audited on an older version and chosen because it was French and they applied for it (French) https://www.numerama.com/tech/1575168-pourquoi-les-ministres-vont-devoir-renoncer-a-whatsapp-signal-et-telegram.html
Google translate link original post : https://www-lepoint-fr.translate.goog/high-tech-internet/les-ministres-francais-invites-a-desinstaller-whatsapp-signal-et-telegram-29-11-2023-2545099_47.php?_x_tr_sl=fr&_x_tr_tl=en&_x_tr_hl=fr&_x_tr_pto=wapp
The translation has some mistakes but good enough to understand the context.
Here is a short summary :
Olvid passed a 35d intrusion test by Anssi (French cybersecurity state organisation) experts or designated experts, with code examination without finding any security breach. Which is not the case of all other 3 messaging apps (either because they didn’t do any test, or because they didn’t pass).
This makes WhatsApp, signal and telegram unreliable for state security.
And so government members and ministerial offices will have to use Olvid or Tchap (French state in house messaging app).
More detail in the article.
Signal’s official policy is that third party clients aren’t permitted, and lacks reproducible builds for their android client. Even if the open source code was up to date, using it without patching it to use a custom server would be a TOS violation.
One of the ways Signal doesn’t really feel FOSS that I read about was related to third party clients and the official server. Projects wanted to use forks of their client with the official servers. In one case this was just so they could remove nonfree software. In another they were adding minor features (that Signal would have been free to take back into the main build, since they were under the same license). But Moxie said they couldn’t use their servers, period.