Eh, for retaining muscle density & mass I'll defer to sports medicine tradition and what I learned from trained coaches there—

Healthline also posts articles saying it's important to eat right after exercise, so I gather they aren't speaking unilaterally in either case:

https://www.healthline.com/nutrition/eat-after-workout

Yea, I'm not trying to argue that eating right after exercise is not important. I just don't believe it's essential. Your body will use nutrients from the post-workout meal to help rebuild muscle and help your body recover so it's highly recommended. If muscle mass and density is a top priority than you may consider it essential for sure.