You're equating two different things here: "building a custom web application" with "quantified self". These are two entirely different things.

Felix's statement isn't a condemnation of quantified self. I think that sometimes, when you're applying algorithms that aren't well-studied, you get pointless or bogus data. I feel much this way about sleep tracking algorithms from the likes of Apple and Whoop. Vitals, too, although it seems remarkably good at detecting when I'm about to get sick.

As a person in the older demographic of HN, having an Apple Watch quietly collect useful data over years and store it in a database has been immensely useful in helping resolve medical issues.

I used to really be into QS, and if I'm training for a marathon, I'm still studying graphs and drawing conclusions. I'm still into QS, but now I just silently log data to Apple Health and use it for medical histories or to identify certain trends (vo2max, cardio fitness, blood pressure, etc) a couple times a year.