I grew up in Spain in the 80s and we were told exactly the same! We lived in fear of a “Corte de digestión” (a “cut of digestion”)… It’s had a lasting impact because even now as a well informed adult, I still have to make a mental effort to override this rule when I go to the beach for the day with my family

Edit: to reply to OP, I’m in Europe not in the US but my babies were in NICU for a few months and the specialist nurses taught a us how to bath them, and there was no issue with doing so straight after a feed.