It's only true if your baby is nursing so long because baby cannot transfer milk effectively. Since your baby is not nursing with normal frequency, the milk transfer may be fine and baby simply needs to nurse more often.
Whether or not to give formula to your baby is a medical decision you should make with your doctor, formula is a medicine for treating specific issues and nurses should not be pressuring you to do anything.
I strongly suggest, start nursing
more often and see an ibclc to have latch and milk transfer assessed. If a baby is not gaining as they should by breast feeding alone, that means baby is not getting enough breastmilk for some reason. The question is why. Supplementing only treats the symptom, usually while causing other issues. Sometimes it is needed, of course. But far more often it is given unnecessarily. Even if it is needed, the cause of the problem still needs to be identified and fixed.