To clarify, He mentioned 1500' as the assumption on the diameter of the Split S to leave yourself, depending on your aerobatic "floor". Starting a Split S at 1500' AGL is surface-level aerobatic territory, which requires the relevant waivers and hopefully the ability as well. 1500' AGL is the legal, non-waivered aerobatic floor. So you would want to stay at least 1500' above the 1500' AGL aerobatic floor (3000' AGL) for a Split S.
And 100KT is a just his recommended number that's fast enough to get decent roll rate without snapping out of the half roll, but also leaves you with a margin under redline at the bottom if your technique is less than perfect. There are no sacred numbers in aerobatics - you can do anything across a fairly wide range of airspeeds, depending on technique, ability, how much you want to stress the plane, and what kind of margin you want to leave yourself. With experience, you'll decide what kind of entry airspeed numbers for various maneuvers work well for your style and comfort level.
Many folks worry about getting as slow as possible before rolling for a Split S, due to concern about airspeed as the bottom. Airspeed on the Split S is controlled by immediately loading the plane to relatively high G after rolling inverted. This will keep the speed down and make for a better looking figure than trying to get as slow as possible before rolling, and then letting the nose fall through slowly, since you really can't pull very hard at such a slow airspeed. Up to a point, you can do a Split S with more speed than you think, but you need to immediately get the G on, also be able to do a half roll to inverted without losing altitude. For most folks in an RV, and without inverted systems, it's going to be a pitch up and roll from fairly low airspeed before pulling through, rather than a level half roll and a perfectly round half loop down competition type of figure.
