How is he far behind. He already has the same # of slam titles on hard courts as Lendl and only 1 less than Agassi at only 25. He already has a ton of Masters titles, 2 WTFs (which is always played on an indoor hard court these days), and many additional slam finals and semis. He was far more dominant overall on hard courts than Agassi ever was apart from maybe 1995, and even then Agassi wasnt able to win the U.S Open due to Sampras, and his dominance at the Australian Open far exceeds what Lendl was able to show at either slam (going 3-5 in U.S Open finals is not dominance, sorry). Furthermore most now consider him the Australian Open GOAT, which alone elevates him to a high ranking.
Now regarding Lendl I think a large reason I ranked him where I did is I flat out think all those others were better hard court players and prime to prime would all beat him more often than not in a head to head. Keep in mind Lendl didnt win squat on hard courts until McEnroe fell way off form, and Connors was 32, despite being top 3 for years at that point. Connors at 30 and 31 beat Lendl at the U.S Open and generally was still dominating their rivalry. As for the what ifs about the Australian Open, one could make just as many about Agassi had he played it all those years before 1995, and one could point out he almost certainly won the 1990 Australian Open only due to Edberg's injury in the final, etc...Note what Connors himself said of Lendl at the 1992 U.S Open: