What is the validity of the saying that whoever catches the second رک'ah of Friday prayer must complete it as four رک'ahs, on the grounds of not having heard the sermon, which takes the place of two رک'ahs?

There is no relation between comprehending the Friday prayer and it being two Rak'ahs. Imam Al-Shafi'i stipulated that the Friday prayer is attained with one Rak'ah, to which the worshipper adds another Rak'ah. As for it being two Rak'ahs, this is because the Khutbah (sermon) takes the place of two Rak'ahs.