If that was his genuine concern, then instead of trying to balance a portfolio of 103 stocks... you simply buy QQQ and short Tesla at 3.53% worth of your QQQ holdings.
And if we want to talk about "bad financial advice", I think telling people to try and time the market with a short is considerably worse than "buy the same shares that QQQ does".
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).
I'm unsure what SpaceX's weighting would be in QQQ but with Tesla being <3.54% weighting it would take both companies being 0s within a year to offset the cost in taxes from reweighting...