I think it was somewhat reasonable for the author to assume it would be a 10-minute job when it had been that way in the previous places he lived. Realizing that a brand-new house might be "unfinished" in some ways is something you might not know if you've only lived in places where there was a previous owner that took care of all the unfinished bits.
But I agree that once the author realized he needed to go to the hardware store a second time, he should have gone through the motions of pre-checking every step he'd need to perform so he could see what else wasn't right.
"I think it was somewhat reasonable for the author to assume it would be a 10-minute job when it had been that way in the previous places he lived."
I don't - in all of those, the author knew someone else had done the work for them.
It's like always having had scaffolding built for you, and then when you have to do it from scratch, assuming the scaffolding required zero time and zero knowledge to build, and being surprised when it doesn't.
Here's a fun thing that most software developers never do that works, and would have also worked here: He could have asked.
"Hey random internet people, i'm about to install a washer in my new home. All of my previous homes i was not the first owner, but here I am. What do i need to think about here?"
A quick search shows lots of people have asked on reddit and get correct answers for their situations. I don't see one where they got bad advice.
The fact that software developers assume it's reasonable to not bother to seek knowledge when you know the situation has changed says more about software development than it does about reasonability :)
But I agree that once the author realized he needed to go to the hardware store a second time, he should have gone through the motions of pre-checking every step he'd need to perform so he could see what else wasn't right.