You measure the charge (if it is there) before it is completely gone (if it were ever there). Capacitors always leak, these capacitors leak fast.

Measuring the charge also removes some of the charge -- fast, compared to the leak spead.

DRAM chips have a circuit that writes the value back -- charges the capacitor up if there's supposed to be a charge, drains it if there's not supposed to be a charge.

Refreshes and normal reads are the same, except that normal reads sends the value(s) to the output pin(s) on the chip.

He has "only" shown the basic grid of capacitors and transistors. The chip he shown has no circuit to measure charges or to write them back afterwards. This makes it easier to test the basic grip and the basic capcacitors.

Pretty sure the proper read out and write back circuit comes in the next video.