"This'll take an afternoon" - three weeks later......
Programmers are notorious for this.
BUT even apart from this problem ... you absolutely should use every dependency you can that will save you time.
Try to write less code not more. When you write code you write bugs, add complexity, add scope increase need for testing, increase the cognitive load required to comprehend the software, introduce the need for documentation..... there's a vast array of reason to use existing code even if you truly could estimate it and build it in an afternoon.
You also assume that you understand all the edge cases and fickle aspects of the dependency, all the weird ins and outs that the dependency author probably spent much resources understanding, fixing and bug hunting.
There's a hard fact that proves the above poster to be wrong..... how many dependencies took only an afternoon of time in total to write? Hard to say (maybe look at the github commit history) but I'd guess almost none. It didn't take the dependency author an afternoon, so why will it take you an afternoon?
Even worse .... you just lost an afternoon coding features for your core application.
Multiply this by every dependency that "you could build in an afternoon" and you'll be in Duke Nukem Forever territory.
I'd advise doing the opposite of this articles suggestion.
Find a dependency that will save you an afternoon? Grab it.