>lean muscle mass gains for groups that consume animal based proteins

The difference in the study you linked is very small, to the extent that it is only really relevant if you're a professional bodybuilder. The average gym goer won't see much of a difference. If they're really worried about it, they could supplement their diet with protein powder (though it won't be necessary).

Its much more important that you have an optimal training regimen than worrying about where your protein comes from.