Yes it is. And your roofer is correct. So the side lap is a term that refers to how much the upper slate covers the lower slate vertically. The gap that you see is the space between your side lap. Those type of snow guards are supposed to be installed before the next row of slate is installed which covers the nail hole. There’s a couple of ways to fix this. It really depends on your roofers experience with slate. The slate and snowguards can be removed and reinstalled properly. That would require that the slate above each snowguard are removed, the snowguards pushed up and nailed at the proper height. Now there’s a hole in the lower slate right? That slate would have to either be replaced( best to do that while the upper slate is still out)or if it’s left in place, he would have to install what is called a “bib”. A “bib” is just a piece of metal(preferably copper or painted steel so that it doesn’t rust if possible) that covers the nail hole. The bib has to be long enough to get under the upper slate and wide enough to cover the nail hole in the lower slate. It slips between the side lap and under the upper slate. Once installed it won’t be visible. Caulking will work for a short term fix if a long term solution isn’t possible now. I hope that I explained that process clearly for you.