That is the rule, yes.
Let’s say you touched toes in bounds, slide the toes out of bounds, then lift the foot. That’s a touchdown, because the entirety of the step happened in bounds.
Then let’s say you touch toes in bounds, slide the foot out bounds, heel down out of bounds. That’s not a catch as part of the completed step happened out of bounds.
now let’s say toes down inbounds, heel down in bounds, whole foot slides out of bounds. Good catch because completed step was all in bounds before you went out.
Heel in bounds, no toes touch, lift it as you go out. Good catch, completed step was all in bounds.
Heel in bounds, then toes out of bounds noticeably later. No catch, completed step wasnt all in bounds, even if it was a delayed step