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) numerama.com/…/1575168-pourquoi-les-ministres-von…
Google translate link original post : …translate.goog/…/les-ministres-francais-invites-…
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.