Other than the one on the front deck the rest of the chains visible in your photos go to either a chain pull or through the rail and under a spool/pocket before hooking back to the rail. Is there a law somewhere that says you can’t hook directly to a rub rail with a chain? I hook straps directly to the rails all the time.
Would I do it on my trailers? Of course not because I know it can cause the hook to dig into the rail and possibly bend it. This guy will learn that eventually.
Personally I’d be more afraid of all that weight sitting on the outside 6″ of the deck. Hope his supports and cross members are strong. Again not illegal. It will likely bend/break eventually.
Looks like he has at least 8 direct 3/8″ chains so he should be perfectly legal on securement as long as that thing doesn’t weigh over 52,800 lbs.