The most important time to apply the advice from the article is after you have decided a project is worth your time and before you start planning in earnest.

The examples of difficult subject matter the author gives (particularly math) are so intimidating to some people that they miss out on a lot of great stuff because they conclude "I could never do that".

I don't think the author wants people to make poor time estimates and suffer budget overruns.

To me, the message of the article reads: "If you think something looks worthwhile but difficult, take a stab anyway. Often the hairiest, prickliest parts of the undertaking from an outsider's perspective aren't that tough at all once you're in the thick of it"

I 100% agree with you about how valuable conservatism is when you're planning, but I think the author's encouragement is intended for people who haven't even considered planning yet.