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

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

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.