Absolutely. It is a clearly written rule. Why does it matter? Quick pitching for sure.Actually it does impact the hitters. The pitch must begin with the hands coming together. It’s what keeps pitchers from quick pitching. It’s the essence of that rule.
I called a similar situation in a HS varsity game last year in May. Actually was pitcher getting on the rubber with hands together. This is illegal in NFHS. Coach says its never called. At some point the pitcher needs to learn the correct mechanics; it may be in 12U, JV Varsity, 16U showcase whatever its illegal and will be called at some point. Calling IPs only helps the pitcher grow. They made a pitching change and lost the game. I had them again 2 weeks later and guess what? The same pitcher got on the rubber separated every time and was ready for postseason.
For the good of the game the rules need to be enforced.
My DD plays lower level B and IP's are rarely called. Go to a showcase and everything is called. We might as well teach them the correct way.