What if they only sort of peripherally remember the name? It's about finding it, rather than using it.

Most people (even geeks) use google to find a site they don't remember the exact url for, and google does a very good job at finding that url, regardless if there is a .com with the same name.

Eg. searching for 'python' gets you python.org as 1st result. You don't want to go to the .com

Because a ton of people link to python.org. Your startup most likely will not have that advantage.

Also, I wonder how many people (say, someone's boss), heard about this cool Python language and just tried python.com. Oops.

Single data point: I had not seen python.com before this, and have been programming in Python for a few years.

Somehow I also remember it's php.ne, not .com, even though I haven't visited that site so much lately.

But both of these were already an established internet brand when I started using them.

Well, Gruber has daringfireball.net and .com