Ι definitely disagree with that. Sure, focusing more on grappling and less on wrestling and striking could make sense. You can do, say, 4 grappling sessions per week and only 2 striking and wrestling sessions each. But doing zero striking for a few years, while focusing on grappling... there are many good reasons why that is not ideal. Imo, periodising your focus would make the most sense for someone starting out. Spend a few months focusing on grappling while doing, say, 1-2 sessions of wrestling and striking, then spend a few months focusing on striking while doing, say, 1-2 sessions of grappling and striking, and so on.