He uses "high skill cap" meaning "high skill capacity". That is, high skill floor, high skill ceiling. Think of it as a really big cup... It can hold a lot of water, but it's kind of dumb to use a huge 32 ounce cup to drink shots.
Likewise, "low skill cap" is "low skill capacity". Low skill floor, low skill ceiling.
A lot of champs in LoL generally have this "high, high" or "low, low" pattern. Eg, nearly every tank is "low, low", Assassins (when they aren't OP) generally fit into "high, high". Champs that are "high, low" are pretty dumb to play, since they are hard to pick up but once you achieve basic mastery you don't really get much better with them...
The only thing that is left is "low floor + high ceiling". In LoL, i think that is mostly generally safe, medium-high range, skillshot-reilant AoE mages such as Orianna, Ziggs, etc. Champs that have a pretty low barrier to entry (once you get blue buff level 7, laning is easy and you just get 6/6 CS every 30 seconds), but there's a huge difference between an Orianna who has perfect ults, shields, QWs and one that doesn't...
Anyway, since pacov doesn't play a lot of midlane... No need for more complicated terminology. 
i think this is also approaching from purely a "mechanical" perspective, not a decision-making perspective. While champs such as Lissandra and Gnar are not really mechanically intense, there are other decisions those champs have to make, such as making clean initiations, who to ult in teamfights, whether to dive or peel, etc.
