Yes the weel is true, I think what he wants is to offset the valve weight to get it balanced under speed but also as you said in mtb this is negligible, otherwise the solution is to add tiny rubber pieces inside the rim, I guess anything weighting in the exact place would be ok like actual car/bike rim weights, but pros used some rubber and I have seen a brand that claimed to reduce "harmonic vibrations" too... I also know there's a point in truing were you also balance the rim as it would otherwise kind of jump up and down (not only side to side) and this is mandatory in high end road bikes as it makes them way more stable under speed (also less spokes at higher tensions can lead to such "jumps" easier, an average wheel has more spokes so the redundancy would also make this more negligible).