actual limitations of locked bootloader
Your statement is correct. :) The bootloader can't prevent images from being flashed, it just prevents unsigned images from loading. My answer to this question has some detail on the way locked/unlocked bootloaders work.

For example, if we consider "SP Flash Tool", an application that communicate with the bootloader of MediaTek devices to flash images, the lock status of the bootloader does not affect ability to flash. (It is still possible to block flashing by only allowing signed binaries to communicate with the bootloader) But still, if an unsigned image is flashed, the bootloader will refuse to load the image.

Still, usually images are flashed using Fastboot (even when there are more options to flash like "SP Flash Tool") and quite a lot of devices only have Fastboot as a known flashing method. Some common exceptions include Samsung, and MediaTek devices. And Fastboot checks most of the time if the bootloader is unlocked, and only lets flashing images if it is. (Same goes for Odin, the flashing application for Samsung devices. It only lets flashing when bootloader is unlocked) So, unless there is a secondary way to flash, it usually is not possible to flash with a bootloader unlocked.