It's just plain wrong because I've never seen a single professional mixing engineer be scared about the amount they're boosting or cutting.
I've been using EQ for 15 years, working as a live sound engineer, a broadcast recording engineer, and a freelance mixing and mastering engineer.
And since 2009 I've been helping people like you turn your amateur demos into professionally produced records you can be proud to release.
The worst thing I see is when students are so indoctrinated with garbage advice like this that they get scared to approach their processors.
Take my student Isobel for instance. She's a retired attorney from the U.K.
(I think they call them solicitors and/or barristers there...which are objectively more eloquent terms).
She's been mixing her son's symphonic metal album.
(Which obviously makes her mother of the century).
And the biggest leap she made with her mixes is when I told her to go overboard with her processing.
And when she stopped caring about arbitrary dB numbers and just cranked stuff up to eleven her mixes instantly improved.
i couldn't believe the sound of the next mix she brought in. The guitars screamed out of the speakers and the drums hit you hard.
That's what happens when you stop worrying about what you "should do" and just do whatever moves the needle of your mixes.
This "no-boost rule bullshit" WILL stand in the way of your mixes sounding better, which is why my Audio Issues EQ has an entire module dedicated to boosting the frequencies that enhance your mix.