Outside of pitched battles, non samurai weren’t allowed to hold weapons much less swords. Peasant levies could be called on for the defense of a land, but they were just temporary troops.

Retainers and bodyguards defend the title of their lord, and were essentially always of the samurai class. If you were recruited as a peasant for the job, then you became the class.

It’s not like medieval Europe where you have to be officially knighted to join the class. The equivalents of man at arms and mercenaries were also of the samurai class.

So as a cross poster said: the man was officially taken as a bodyguard to the daimyo, received a household, and was presented with a katana. Thus for all intents and purposes he’s a samurai. There’s just no other class permitted to do that job.

Edit: It would also be a grave insult for a samurai to be protected by a non samurai. To claim the lord, a noble warrior, requires a mere peasant to protect him… well that’s not permissible. So the daimyos bodyguard will be recognized as a samurai at least within his own lands.

I believe who counted as a samurai was much less clear during the Sengoku period, to the extent where holding a sword did basically make you a de facto Samurai. After unification, those following the major leaders were the only ones recognized as samurai.

The rigidity of the samurai as a class varied largely depending on when you're looking in Japanese history. In the Heian Court period, samurai were just armed people in the service of Japanese nobility (deriving authority through relation with the Emperor who was considered to be descended from God.) After the Genpei War the samurai became more hereditary until the Sengoku/Warring States Period.

Several of the key players during this time were themselves just peasants. Then after Tokugawa took over, he once again encouraged the hereditary samurai as a way to ensure stability.

Yasuke was under Nobunaga who was very much active when the definition of samurai was much more fluid than rigid.