Kasparov couldn't have picked a much tougher field than this... If he doesn't beat all these World Champions in blitz (four of them in the field, and that doesn't include Nakamura, Nepomniachtchi and Anand!) people will say that he didn't impress at all and is overrated. Even if he would win (which he won't), people would say that, well, without Carlsen in the field it proved nothing.
Even at his peak Kasparov was by no means the clear favourite in speed chess events. Back then blitz wasn't as big as it is today. Now everyone is playing blitz all the time, and the level is higher than in the old days. And if Anand could do better than Kasparov in speed chess when both were active, it won't be all that easy for Kasparov after being retired for many years. It's just to look at Anand's score in the latest event to see how tough it is today.
My guess is that Kasparov will end up with an even score in the middle of the field, and that will be a good result.
Magnus wasn't scheduled to play in Sinquefield. That may have been part of why Garry chose that event!