Nice diligence here.

A few comments.
1) I wouldn't say 6% rewards are "too good to be true" when BlockFi and Celsius offer 8% for staking. However, it's very competitive and the best for a crypto card by far.
2) The 6% requires a TERN stake which is why I'm more confident that it's related more to staking then orderbook trade. For example, I stake 145,000 TERN and that costs me $1200 which drives the price up. If i plan to keep using Blockcard its a great deal because I can profit from others staking but earn rewards short term as well.
Staking TERN at this level makes a lot of sense as it's relatively cheap.
3) The biggest issue I have with your math is that it does not accurately reflect the conversion. I can go buy TERN on Whitebit or another exchange at 0.07 and deposit it directly on to the card. As long as I spend at 0.07 then I'm always good, regardless of the orderbook.