Maybe, as an outsider, he didn't know how did those protests go, if most were peaceful or not, it really depends on how you receive that information. I also remember seeing quite a few videos showing violence associated to those BLM protests.
I live in Catalonia, Spain, we have had some political trouble here, including protests, the overwhelming majority peaceful and depending on where you see that information it just seems it is the opposite. In fact a political party now that we are a week away from voting , is comparing in their tv spots, those peaceful protests with the pro trump attacking the American institutions.
My point is that he really didn't justify anything, he just did a bad comparison, and people are really judging and defining him without further background or proof. In fact that is a very Trumpist way of doing things, imo.