https://droitromain.univ-grenoble-alpes.fr/Anglica/D1_Scott.htm

4. Ulpianus, Institutes, Book I.

Manumissions also, are part of the Law of Nations, for manumission is dismissal by the hand, that is to say the bestowal of freedom; for as long as anyone is in servitude he is subject to the hand and to authority, but, once manumitted, he is liberated from that authority. This takes its origin from the Law of Nations; since, according to natural law all persons were born free, and manumission was not known, as slavery itself was unknown; but after slavery was admitted by the Law of Nations, the benefit of manumission followed, and while men were designated by one natural name there arose three different kinds under the Law of Nations, that is to say freemen, and, in distinction to them, slaves, and as a third class, freedmen, or those who had ceased to be slaves.

Ulpian, quoted here, was active around ~215 AD, and like most Roman jurists, was involved in expressing pre-established legal principles of Roman law.

Florentius, who your original quote cites, is a 2nd century AD jurist, for that matter.

I know Gaius, another 2nd century AD jurist, stated the same thing, but I'm trying to dig up the exact quote. I might give up, tbh, I don't feel like pawing through my actual books to find the citation and search engines are just slop anymore.