That's definitely not how it is.
Khabib and his coaches are extremely knowledgeable in martial arts, so obviously they would know about the fact that training offence all the time can come at an expense.
Plus when u work your ground game on top your bottom game will get better. Its like when you're learning how to defend submissions, you cant just learn how to defend them you have to know how to apply them to properly know how to defend them
Plus look at how many great fighters they have in the gym. There's no way an elite UFC champion does not practise his bottom game especially when he is a ground based fighter