Break down the timeline logic. The key word is "later".
AD released today.
Last inspection may or may not have been done 50 hours ago, which pre-AD was voluntary or part of a non-mandatory maintenance program, or part of some recommended Service Bulletin, that is now being mandated, or maybe it was never done at all. Assume it was done 50 hours ago.
Which requirement is later? 25 hours from today, or 100 hours since the inspection done 50 hours ago? 100 hours from the last inspection is still 50 hours out from today, so it is later than 25 hours from today, so do it in 50 hours.
Say last inspection was 90 hours ago. Hundred hours later comes up in 10 hours. But the AD says whichever is LATER. 25 hours from today is the later than 10 hours from today, so that applies.
Or inspection has never been done before. 25 hours from today.
In no case do you have to do it in less than 25 hours. It's more or less the same as that other one you posted about landings, but with a bit different wording.