I sometimes get the feeling, also by watching their public content on YT, that a lot of YC advice, especially w.r.t. what to prioritize, is from an era of startups about 10-15 years ago.
Back then there were about 10x fewer SaaS startups (maybe that's a conservative estimate) and if you had a decent product that solved a problem for your customers and you were just plugging away at it, that was enough to get noticed and grow.
Nowadays every worthwhile niche seems to have 10-15 companies in it in no time, all of them doing more or less the same thing. Even if your company differentiates in a very special and useful way, getting that across to your potential customers is a totally different game now. You basically need 1-2 full time staff doing nothing but content creation and social media outreach marketing. Also locking in (micro-) influencers in your niche is key. Once they start promoting your competitors it's game over.
From the content that I see publicly on YT etc. it seems to me that YC thinks such things are basically a waste of time. And back in the late 2000s and early 2010s that was largely true. If you are targeting a narrow hyper-niche it is probably still true, however it's just not the reality for 99% of today's startups, even YC ones.