It's not clear to me exactly how the salt token figures in the loan arrangements. Their white paper ('abstract') identifies the role of the blockchain in recording transactions, facilitating smart contracts that action the loan agreement etc, but I don't see where Salt is used for payment, other than for some kind of membership fees.
Perhaps I'm misreading their publication, but I don't think the arbitrage you identify would work in the way you suggest.
It seems like it's worth signing up for at least, and hodling
In the interests of self-promotion, here's a referral link:
https://membership.saltlending.com/register?r=TDS84
They wont except SALT tokens to pay down principal like they are now forever!! Just to lower interest!! But to incentive people to take loans they are currently allowing it