They say (I've always wondered who gets to designate the 'they'. . .) that the first million words a writer writes are for practice--that it takes that many words to hone the craft. I don't know about the exact number, but I agree that to get better at writing, the writer needs to write.
Now, some of those words are going to be in a specific genre. Just as in athletics, there is likely some specificity in training effect here. If you want to get better at writing YA fantasy, don't practice writing police procedurals. (Yes, I've often been heard to say 'writing is writing' and I still believe that, to a point. Each genre has its conventions and you must learn them, know the cannon if you will, before you can place your own stamp on it.)
I have close to a half million words of novels written at this point, spread out in 4 complete novels and one about 1/2 to 2/3 completed. But I also think that writing up critique for other's novels can and should be part of those million words.
We all get too close to our words and develop horrific blindspots. We tend to make the same mistakes over and over again, until someone points them out. That would seem to be an argument for obtaining good critique from a writer or group of writers you trust. And sure, getting an excellent critique is extremely helpful. But so is giving excellent critique.
Providing critique hones your analytical brain. It helps you learn to figure out *why* something isn't working in someone else's work. That someone else is far enough out of your own headspace that you can have the necessary objectivity to assess the work. You learn to see patterns and that skill at seeing patterns is something that you can then turn to your own work.
In almost all the critique I've done, there comes a moment when I see something that exists in my own writing. Because I see it in someone else's work first means I can get over being defensive about it in my own.
The process of reading for critique is essential in troubleshooting your own work. And it will make you a better writer.