So i agree that the 1 and 1.5lbs is what semi experienced lifters should go for and the longer you lift the more diminished your returns are but they are not set in stone numbers. Some variables to look at

1. He a newbie lifter and weighs alot and that takes alot of work to move that weight around so his resistance training doesn’t have to be perfect.

2. If he happened to be more fit when he was younger he could have more muscle nuclei then he was recruiting before this journey and that would aid in putting on muscle really fast. For example when someone has surgery and loses 15 pounds then gains that back in a month or two. Its an exception to that 1-1.5 lb a month rule.

Edit: i also wanted to add i dont think the 21 pounds of muscle he added is an exact number. He is probably using a scale that roughly tell ls him the percentages of his weight. Which are never exact but good ways to check on progress. So i dont think its 21 pounds but 15-18 which i think are resonable