If this is the hardest question for you, you are not ready to do a startup. This is a filtering question - If I'm YC, if even one of the founders is not able to commit, that's probably an immediate rejection right there.
That said, if you aren't at the stage where you can commit to doing a startup, you probably shouldn't be applying to YC.
I believe it wouldn't be an immediate rejection but a strong warning indicator. If a founder is not willing to commit to their startup full time, the startup is going to fall apart when all hell breaks loose. Founders that are half committed may have strong disagreements with each other when there is a conflict of interest (eg: too much time spent doing his full time job while working on the startup). Disagreement between founders can kill a startup.
I suggest the author to find a full time job or work at another startup first. Do a startup when the time is right and you saved up enough money.
If you decide do the startup, don't look back. Be committed to your decision.
I do see where you reached that conclusion - thanks for the feedback. I'll try and be more clear in future articles.