I think what Swan mainly means isn't necessarily part time or not...
but complete and total commitment to getting it to work. Am I getting that right BS ?
If you have a good salary etc will you really be disciplined enough to always do what needs to be done, or will you get negligent because you don't really need the money.
It's like someone who inherits money, for them it's all a game until the money runs out right.
I wouldn't say it's impossible, but you definitely need total commitment and discipline.
In a way trading with a safety belt (salary, inheritance etc) is a bit like the difference between paper trading or going live, the psychological pressure is different, and psychology is imo 90% of trading success.
I wouldn't say starting out part time is impossible, but it can definitely only work if you take it just as seriously as if you were trading for a living, and most won't be able to generate that discipline without outside pressure like a boss telling them what to do or inside pressure like having to pay bills from your trading.
After all most don't have the discpline either to keep their weight down as long as there's no pressing problems cropping up like diabetes etc.
Even then most won't develop the needed discipline to do what needs to be done but keep eating sugar etc, and similarly that's why imo it's hard to take trading seriously as long as you're comfortable financially independent of trading.
Not impossible, but impossible without taking it 100% as seriously as if your bills had to be payed out of your trading account even if you still have a job etc, I think that's what it's all about.