The dust threshold you are referring to here is the payout dust threshold of $0.02. This value is irrelevant for curation trails. You can vote with lower values and still increase the author rewards of a post or earn curation rewards (given that the sum of all votes exceeds $0.02, obviously).

What's relevant for curation trails is the dust vote threshold at 50M rshares, equivalent to a 100% vote at 1.2 SP. If a vote is worth less than that, it is recorded with 0 value, does not increase the author rewards and is not eligible to earn curation rewards. This value is independent of the Steem price.