There is another variant of the Flex Cap approach that allows miners to pay with a slightly higher difficulty target instead of deferring a portion of subsidy to later blocks.  I think the HK presentation was about the subsidy deferral variant because of miner feedback that they preferred that approach.

Myself and a few other developers think proposals like BIP100 where the block size is subject to a vote by the miners is suboptimal because this type of vote is costless.  You were astute in recognizing in your post it's a good thing to somehow align the global marginal cost with the miner's incentive.  I feel a costless vote is not great because it aligns only to the miner's marginal cost, and not the marginal cost to the entire flood network.  Flex Cap is superior as "vote" mechanism as there is an actual cost associated, allowing block size to grow with actual demand.

Warren