That can be restated as: They do not know a limit so they set the limit to zero. No surprise there. Anything else could be dangerous for the fetus.
Sometimes old wives tales turn out to be correct.
That actually makes a lot of sense.
My older brother was born in 1972; her doctor advised her not to quite smoking because the stress would be bad for the baby.
They have been saying that for at least 40 years. (Actually what they said was, wear gloves. But I let my husband do it and he didn’t mind. He still is the major cat-box cleaner and the baby is 23.)
I’m 33 and my mother still has my father doing it.