Muscle growth and muscle breakdown are separate processes that are both constantly happening in your body all the time.
When you workout to stimulate growth, and eat plenty of nutrients, your growth rate gets faster. When you don't use the muscle, and your body is short on nutrients it'll find nutrients from stored sources of glucose, protein, and fat, and so some muscle gets broken down.
What matters to you is growth - breakdown = net change in muscle. So yes, being in calorie deficit means your muscles breakdown is high, but if you are working out consistently, you also grow, enough to replace most of the loss, and if you're new in the gym you grow fast enough to outpace the loss entirely.
However, if you're not new in the gym like me, your muscle gain rate is diminished and so intermediate and above lifters should not expect recomposition to work. That is why intermediate and above should not recomp, and should instead bulk and cut if they want to optimize net muscle gain.
However, the majority of the population will benefit from recomposition. If you're skinny, maybe jump straight into bulking. If you're overweight, maybe more calorie reduction is called for if the fat is so thick it will overshadow the muscle gain. However the difference between bulking/cutting, and recomp is mainly a question of optimization, not effectiveness. Because both approaches definitely work for the majority of people, and what is most important is consistency, so that means making comfortable, reasonable lifestyle changes that can be sustained indefinitely to produce a lasting long term result.
