Its not particularly useful to just parrot the information without also including their justification; not all of us take YC mantras as the gospel.

Did they give you a reasoning for this guidance?

You don't know your LTV, churn rate, or even if you have product market fit at the beginning so paid acquisition is sort a gamble. I think YC soundly advises that you get your initial customers in ways that are less scaleable and get to know them a bit before you turn on the firehose.

I have not attended the Startup School, but I imagine there's a lot of value in personally recruiting your first 20 or 50 customers. You get to talk directly to them; you get a chance (not easy, though!) to find out the real reason they stopped using it, etc.

I imagine you can talk to people that see your ads too.

Going after your customers looks great (honestly, I have zero successful experiences either way), but it is really hard for people that have a day job.