Okay, I'll confess: it's me asking. This was totally my topic suggestion because this advice really irritates me - I think it's wrong, even dangerous - and I want to know what everyone else thinks about it.
So, I feel a little bad kicking off this topic, as I know I'll be setting the tone here.
The reason I think this advice is flat wrong is threefold.
First of all, the process of writing a book takes HUGELY longer than reading a book. Let's say it takes 5 hours to read a novel. (It seems like my Kindle often reports something in that neighborhood when I open a new book.) I'm a fairly fast writer, and I keep track of my numbers, so let's use my writing time for an example. It takes me, on average, 55 working days to draft and revise a novel. In general, I spend 3 hours/day actively writing to get that book finished. That's 165 hours of writing, at a conservative estimate. That means it takes a reader approximately 3% of the time to read what it takes me to write. And I'm a fast writer! The percentage will only go down from there.
Put another way, a reader will read at least 33 times as fast as we write. Comparing the two experiences is ridiculous, particularly when it comes to a subjective quality of feeling bored, which is time-sensitive.
Secondly, the experience of boredom is entirely subjective. What I find boring is not what you may find boring. I get bored with fight scenes, in books and movies. I know that's a me thing, but they don't hold my attention. Lots of people love fight scenes, which is cool. But there is no objective qualifier of what is "boring."
Finally, writing is a job. It might be an awesome job - it is! - but it's also work, which means it can be a slog. Especially writing novels. Working incrementally for ~75 days (my total time to produce a novel, including days off) on one story can get dull. Some days I'm tired. Sometimes I'm writing stuff I already know, like backstory from previous books, or stuff that isn't particularly exciting, like transitions, but that I know the reader will need to understand the story. I don't know ANY writer who is 100% excited and invested in what they're writing every moment and every word. Sometimes, people, it's going to be boring - and that has nothing to do with how the reader perceives it.
I promise you this. Test it for yourself. Make note of some part of your work in progress that you found boring to write and find out later if any of your readers find it boring to read. I'm sure they won't.
That's why I find this advice dangerous. It implies that only the writing we find exciting in the moment is valid - perhaps even suggests that anything we find boring to write should be thrown out. This is bad for getting words written, which is our primary job. If a section of the story is boring to read when you revisit it? Sure, edit that puppy! That's what revision is for. But don't let feeling unenthused in the moment stop you from moving forward in the story.
Neil Gaiman says that writing a novel is a process of laying bricks in a long road. Some days the sun will be hot, the work mind-numbing, the process slow and grueling. But the bricks have to be laid. Do the work and don't worry about how you feel.