I stand corrected: I should stop replying to messages at midnight.
Indeed, the "Red Hot Rewards" (the 2% on top of basic 0.4%) is for first $100K - though still not enough for most people's rent.
The "maximum $300 RewardCash per phase" (quarter) applies to the separate "online & overseas spending" promotion, which is likely what KeyChain Pay refer to with their 5.6% (I make that $15,000 per quarter at 2%). There's an additional 2% for Premier card holders (also capped at $300) but I don't think it's relevant as that's overseas only.. although the T&C seem to indicate Signature holders would get the 1.2% additional... I don't see mention of a cap for that.
Now I see how HSBC make money in any case; by confusing the hell out of everyone.