"The people who try to stay lean year round are the ones who end up looking exactly the same after years and years of hard work."
Yeah, Shelby knows his stuff.
The other problem with the whole "staying lean vs. gaining bf while building muscle" topic is that people tend to think in extremes.
Someone says something like "you need to be willing to gain some bf if you want to gain substantial amounts of muscle" and people immediately think of sumo wrestlers and SHW powerlifters. Basically people who are obese.
Then, when someone says that they can stay relatively lean while gaining, people immediately think of sub 10% bf levels.
The reality is somewhere in the middle, and will somewhat vary depending on the individuals genetic tendencies.