It is just such a no-brainer for the government to have flexitime for civil servants. It essentially allows them to allocate more work than it is possible for one person to deliver across the course of their working week, but to still deny paid overtime in the vast majority of circumstances because the option to build flexitime is there. You also have some departments, I think, where your flexitime 'refreshes' at the start of each month, so I would imagine there are cases where people just do not have the option to take it all and essentially lose that time, although thankfully I have never experienced that in any of my teams to date.
It was especially useful for me at the peak of the pandemic, because I would have some days where I would be online for eleven or twelve hours. Then, on days and weeks when things were a bit quieter, I would be able to take this time back anyway. As such, I didn't actually mind doing the long days occasionally because I knew I would just be able to claim the time back eventually.
Rees-Mogg is an oddball and actually just a fundamentally spiteful person, and demonstrably nowhere near as intelligent as he or some others seem to think he is.