Your boxing trainer is giving you what we call "broscience". That's the "science" you get from blokes who call each-other "bro", they pass it around between each-other. It sometimes has some original truth in it, but usually not.
It's not that salt is essential to growing muscle, it's essential to life. But added salt usually is not.
Most of us can live without added salt. You need more salt in hot and humid conditions, since when you sweat you also lose salts. Historically people living in the hottest places either had seafood or added salt was very valuable to them. To this day there's a camel-borne trade in blocks of salt across the Sahara.
However, unless you're living in a hot and humid climate, you don't need much added salt, what appears naturally in food will be enough. Exercise will deplete your salts a bit, but you have to be absolutely drenched in sweat before it's at a level where you'd really need added salt.
On the other hand, there is no health reason to cut added salt entirely from your diet unless your family has a history of early heart disease, or you have previously suffered kidney or gallstones, or have other kidney problems. You can certainly cut down your added salt intake - most Aussies have heaps more than they'll ever needed - but most people don't need to eliminate it entirely.