I feel like this is good advice but presents only one side of the coin. Underestimating how difficult something is, is exactly what leads to ''planning fallacy''. I would rather err on the side of caution and overestimate how difficult something is
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.
I think the investing 10 hours in learning a new thing is more than enough to get the initial estimate you describe. Probably even 5 would be enough. That's 10 pomodoros. It's something I'm willing to consider to throw at totally random things to see if they are actually as hard as I think, and if I actually want to learn them.
Most people don't really spend even 15 minutes, to the clock, on things they think are hard to learn - and so underestimate just how much you can progress with as little as few hours, if you actually sit down and do it.