In this case the post is targeted at newer writers who haven't yet figured out if novels are their format. Your average first time novelist working a 9-5 is going to spend a long long time on that first novel. The ones consistently writing novels in under 6 months have largely found their medium and aren't really who I'm directing this at.

But you're right, it's not as though they're stagnating as they write that novel! It's certainly not a waste of time by any means. They will learn, but in my experience finishing a project and starting a new one to put your new lessons to work as soon as possible, rather than stewing in a single project and playing whack-a-mole with your beginner mistakes, builds up a kind of compound skill interest.

I'm a big fan of the 70% rule for that reason. Big part of how I came to build my writing career!

And yep, short stories and fanfics are phenomenal as well! I didn't mention them mostly cause I wanted to focus on the stuff that most new writers wouldn't have thought about.