Okay. I'm going to play devil's advocate on this one, because it will allow me to put forth some recent thoughts I've had about sprinting.
When Matthew Boling burst onto the sprint scene in his senior of high school, he had been training exclusively as a Long Jumper and 400m runner.
He hadn't run a 100m race since 8th grade or before (?). Yet, that year was his best ever year in 100m/200m.
Last year, Fred Kerley, who had run the 400m his entire career, started competing in the 100m at the age of 26, and took silver at the Tokyo Olympics.
Marcell Jacobs who won the gold medal in 100m, had been a long jumper up until 4 years ago.
Allan Wells, the 100m gold medalist from Scotland, had also been a long jumper before converting to the sprints in his mid-20s (26, I think).
Dafne Schippers was a heptathlete, when she burst onto the sprint scene. (When she began to focus only on the short sprints, within a few seasons her times started to decline.)
Karsten Warholm was a decathlete before focusing on the 400m hurdles exclusively a few seasons ago.
My point is, that 'cross-training' seems like it can be quite beneficial to sprinters.
Perhaps the long jump gives them the explosiveness they need for the short sprints.
And perhaps the 400m gives them the strength to pull away from the other sprinters over the last 10 to 20 meters (as Boling did in high school).
So I am not completely convinced that training for the 400m and Long Jump are necessarily bad for his sprinting. They might give him a strength and explosiveness base for later years. However, training for all 4 events - 100m, 200m, 400m, and LJ - is probably too much.