The extent of the closures was always overstated in popular accounts - you say "half", and people often talk as though it was even more, but the highest concrete estimate I've seen is 30%. And in most places the closures have ended.

No doubt, but even so, the markets are barely even down. How so?

I think "barely even down" is a good reflection of the likely impact to the companies that comprise market indexes. In almost all places economic activity has mostly resumed; fears that nobody would be willing to do anything until the virus was gone have largely been disconfirmed. There will certainly continue to be human and economic costs of the coronavirus, but the costs seem unlikely to affect the financial situations of most companies.

Sorry I don't agree.

Not only have whole sectors such as travel, entertainment, retail been directly affected (ie closed), it seems they will continue to be affected by the changes (investments/costs) they need to make to comply with the post-covid world.

The markets have remained levitated by govt. stimulus - the Cantillon effect.

Counterintuitively I can see a case for the rally because things are closed. People are spending less in addition to fed easing and face uncertainty on a personal level while knowing the marker will eventually recover. Better to keep your stock in Tesla than to buy a Tesla right now essentially. With little worth withdrawing their investments for there is less "sell pressure" which helps keep values up.

Ironically this implied an improvement in the general economy could crash values if a demand boom causes more volume of selling demand than buying demand.

Because the government is printing a ton of money, but those companies' value is not changing. That's one component of stock prices: a lot of companies simply have a static value (at best), like houses, but that still has them go up by inflation (the real inflation, closer to the tongue-in-cheek "big mac inflation" of ~3%, not the CPI of 0.7%)