I don't know if that can be true.
"In 1988 Ben Johnson ran a then world record of 9.79 seconds to win the Olympic Gold medal. Subsequent breakdowns of his 'roid induced run timed him as he reached the 40 yard mark. (By the way his times at 50 and 60 meters were faster than the current world records at that distance.)
His time? 4.38 sec"
Source
Ben was always the best starter among sprinters and unbeaten on the first 60m. so i can hardly imagine that powell would have taken 1.3m away from him on the first 40 yds when his endtime is only 0.02secs better then Johnson's.