Having done both full body and split workouts, I've never felt a difference in DOMS between the two, and this isn't something I've ever heard of either.
Some possible explanations I can think of:
- Active recovery: If the additional exercises you added to that session involved your legs and core but to a lesser extent (e.g. anything ranging from bench press leg drive to simply pacing around more between sets), it could help with recovery and reduce soreness.
- DOMS goes down as you get used to a workout. Since you had DOMS at all to begin with, I would guess you've just recently started this particular leg workout. It could just be a coincidence that your body became sufficiently accustomed to it at the same time that you decided to switch to full body. You could try doing just the leg workout next time to see if this is the case.