Thanks RH and DJ
Actually I’m gulity of misquoting Wikipedia, what is written is:
"Since Nijinsky, only Nashwan in 1989 has won both the Guineas and the Derby, and in addition no Derby winner has even entered the St. Leger since Reference Point in 1987"
which can be interpreted as no Derby winner has run in the Leger since Reference Point (true??) rather than not been entered at some point earlier in the season.