The documentation is not very clear on this issue but your bootloader was in fact upgraded.
The bootloader upgrade process does not require a full bootloader upgrade - only a small section of the bootloader code needs to be overwritten - that is why the bootloader version did not change on your switch after the upgrade.
Unfortunately there is no way to upgrade to the latest bootloader in the field from what you already have on there now. The same bootloader code which is put onto to the switch in the factory stays with it.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...