You pay interest on the margin you put up for shorts net profits from the position itself and cash or other assets you place inside investment accounts. You're also usually being charged interest at only a few basis points above the RFRR so this isn't "interest" in the sense of a loan.

> I think telling people to try and time the market with a short is considerably worse

Nobody is trying to time the market. If you want QQQ but don't want the Tesla exposure in it, it's a lot cheaper net to simply hedge against your Tesla exposure with a short position counteracting your long position. If you're worried about margin rates interfering with your profits, you can model all of these and come up with the optimal short needed to hedge your risk. This is standard financial practice.

Shorting doesn't have anything to do with timing the market, the reason why pop investing communities think that shorting and timing the market are synonymous is because as a whole asset prices are expected to keep pace with the RFRR assuming they at least hold their value, so taking a short position is going against the "default" market direction.

The GP did not try to time the market. He suggested a sensible strategy to exclude a tiny subset from an index (less expensive than maintaing the alternative index yourself).

Its not timing the market if it is exactly offset by the position in the etf