Side projects usually don't have the oversight where you learn and improve so much as when you work in a team. Is like learning to play football by yourself by kicking a ball against your house wall everyday, or play in a team with other players and a coach. You can kick the ball for 8 hours against the wall, but I guarantee it that 1 hour a week in a team setting you will improve much more.
I can't believe I have to say this. This is obvious. Every coach knows this.
It's not an exclusive-OR problem.
Boxing for example (sport I did and helped coaching), unless you are top of your field, isn't recommended to do a lot of 'at home' training as you develop bad habits without supervision (dropping the defense, telegraphing your punch a bit) as you don't have the feedback. Thes mistakes have to be then 'corrected'
Boxing isn’t a good example because it’s a combat sport which means there is far more training than competing.
When I played football the ratio was close to 1:1 - games played to hours trained and something like 150-200:1 for when I was boxing.
What’s the likelihood that someone takes their same 10-hour drudgery cycle and doubles down on it by doing the exact same thing in their side project and why does that likelihood round to zero?