The Raspbian images by design support all versions of the Pi, back to the original. The developers have stated they don't have the resources to maintain separate 32- and 64-bit userland images, but provide the config.txt option to at least enable the 64-bit kernel.

If you want a 64-bit userland, there are other options besides Raspbian. You can also try an in-place arch change from Raspbian to aarch64, but I imagine that would break some things, since the Raspbian-specific package repository only has packages for the armhf arch.

I just gave it a try and now apt is installing a bunch of new kernel .img files. So for me, yes, this seemed to be required before 64bit kernels were installed. I'm feeling surprised. I'm 40% of the way through the install and hoping it reboots okay in the end.

Edit: to be more clear, I also run 'apt update' afterwards

config.txt only affects the kernel and doesn't affect which userland is selected.