Silver King pitched 584.2 innings in 1888, and around 490 in each of the following two seasons. The SIM stretches players and pitchers' stats to 162 games, so many of the deadball era players appear to have done even more than they did in reality (though 584 innings is unbelievably impressive enough !).
So Deacon Phillippe of the 1903 Pirates, who was their ace during the first World Series, pitched 289 Innings that year, but is listed in the SIM as having 335 IP/162. The Pirates played 140 games that year, so the stats have been made to harmonize with a 162 schedule. The Pirates used a 4-man rotation that year, though the fourth starter was split between two pitchers.
The St. Louis Browns, with Silver King, played 135 games in 1888, and 71 of those games were started by five other pitchers, 64 by Silver King. Again, his stats were made to accord with a 162 year.
None of this is to take away from the amazing accomplishments of these players (worth remembering though that two years before that, in 1886 millions of American workers went on strike on May 1 to win a 6-day, 8-hour a day workweek. The 8-hour day wasn't won till the New Deal era, so long innings went with the dominant paradigm of long hours). But it is to note that part of what makes 3-man rotations with 300+ IP pitchers so common and dominant in OL play is this stretching, as it were, of their stats to a 162 schedule.
I am not sure why that is not done for hitters - Babe Ruth hit 60 HRs in 154 games (though he had more PAs and ABs than did Roger Maris in 1961, because the 1927 Yankees scored so many runs). Shouldn't his HR numbers be adjusted and be listed as 63 more or less.
There is currently a 700+ pitchers theme league starting, in part to test collectively the best ways to use these 700+ IP pitchers. I will try to get some of the more experienced people in that league to post something here based on the insights gained once the season starts.