I think this is a bro myth, but what I think really happens is that those multi-joint compound lifts are working and strengthening all kind of small supporting muscles almost nobody knows the name of. This core strength then allows you to get bigger/stronger more easily when you do the isolation work.
Negative Ghostrider... not bro-science. This has been established by Soviet sports scientists but also many studies recently in the US.
The more muscle mass you engage, the greater the hormonal response via Testosterone, IGF-1, Insulin, and also Enhanced Post Exercise Oxygen Consumption.
Resistance exercise can acutely increase the concentrations of circulating neuroendocrine factors, but the effect of mode on this response is not established. The purpose of this study was to examine the effect of resistance exercise selection on the acute hormonal response using similar...
free weights provide greater hormonal response than machines ( Compound vs isolation)
Resistance exercise has been shown to elicit a significant acute hormonal response. It appears that this acute response is more critical to tissue growth and remodelling than chronic changes in resting hormonal concentrations, as many studies have not shown a significant change during resistance...
Anabolic hormones elevated during 15-30 minutes of post-resistance exercise providing an adequate stimulus is present. Protocols high in volume, moderate to high in intensity, using short rest intervals and stressing a large muscle mass, tend to produce the greatest acute hormonal elevations.
You have it backwards, the small isolation movements strengthen your weaknesses in a given range of motion. Allowing you to increase the total weight used in compound exercises that then result in greater muscle mass stimulation.
Example: you cant lock out the bench press at 4 inches accross the chest. (mini max)
So you would do Pin presses with a pin set at the sticking point or JM presses or Close Grips for supplemental work. You the build the supplemental exercises with additonal exercises to strenghten their mini-maxes. For instance you would do Tricep Extensions or push downs to build the triceps further in the stretched state. As the accessory movement gets stronger, the new "mini - max" or sticking point increases. Now you can put more weight on the bar and work to a new sticking point... say this time its off the chest... so now you do Fly's/cambered bar benches/paused benches to build the chest, ect...