Vietnam was an unjust war the US prosecuted. The Viet-Cong were deemed the equivalent of terrorists at the time and entire Vietnamese villages (man, woman and child) were massacred by the US army and air force via napalm and carpet bombing. The civilians were dehumanised, soldiers raped and murdered their way across the country.
Military historians later realised that the terror tactics used by the Viet Cong were due from the desperation caused by the asymmetrical warfare of fighting a vastly technologically superior foe that outnumbered them 1,000 to 1, rather than a choice on their part to be terrorist-like.
He is most likely thinking Hamas are just like the Viet Cong, when they aren’t. The Viet Cong never attacked the US, they never expressed genocidal intent against american people, etc.
Americans tend to view the rest of the world through their own lens, ignoring contexts that are not American-centric.
He’s not wrong to support Palestine, but he is woefully projecting his Vietnam experience onto the Israel-Hamas war in Gaza and missing the forest for a few trees.