More buyers than sellers.
Of which Housel says;
This is the equivalent of saying someone has more mothers than fathers. There's one buyer and one seller for every trade. Every single one.
This isn't precisely correct. When you buy or sell a stock or an ETF the other side of the trade is not necessarily taken by an investor. If you have stock for sale, there isn't necessarily a buyer at that moment. The job of designated market makers is to provide liquidity when needed (and maintain orderly trading when needed). On lower volume stocks DMM may need to take in the stock you are selling into inventory and keep it for a while; there was no buyer.
It is also possible that when you sell a stock that the buyer, an actual buyer, is buying to close his short position which is obviously different than buying a stock outright (reasons for closing a short is usually different than reasons to go long).
Housel's reply also does not address that at a given price buyers or sellers will eventually exhaust and the price moves. At $32 there may not be a seller. A market order to buy will go up to find the first available seller.
The other one to mention;
Investors are fleeing the market.
Every stock is owned by someone all the time.
Investors fleeing is probably not very helpful but the implication on on both comments seems to be that there is always a natural participant available for the other side of every trade but the fact is that the market has more moving parts than that.
Kind of tying in with my comments above, if a large ETF is met with a meaningful number of redemptions, it is unlikely that there will be investors lined up to buy, it would instead be participants whose job it is to provide liquidity that would "buy" the shares. Maybe that is what Housel meant but the way I take his comments, I don't think that is what he means.