Just checked if one needs to hold an MD-Pushbutton the whole time during an update.
On my board there is a difference between USB and UART Boot Mode: For USB its enough to have the MD pulled low during reset / power on (with the mentioned cap.), if you release MD and start the flash shortly after that, it works.
In UART Mode the MD has to be pulled low at least until you started the flash process, I released it while it ran and got no error.
Flashing is very fast anyway, would be no issue to hold MD a few seconds, but it's good to know that a pushbutton will do the job: It can't be left in the wrong position and it can be used for user input during runtime more flexible than a rocker switch.