The theory is not based on interest rates, but rather tax loss harvesting.
Hmm… this is different from how I interpreted the advice around investing lump sums versus dollar cost averaging them in. I thought it had to do with the adage that in the long run, time in the market always beats timing the market, meaning one should always invest as much of their capital as early as possible—in the case of IRAs, that would mean January 1 assuming the previous tax year’s contributions are already maxed out.