SAAB:
Of course you are correct for prior to v5.5 dos.
With the advent of ATAPI there are no specific drivers for cdroms as they are done over the IDE interface to the ATAPI specification (unless they are SCSI or specialised).
Therefore MSCDEX was the only TSR required to be loaded in a DOS environment and no .sys driver was required.
p.s. For devicehigh and for loadhigh to work you had to have himem.sys configured as your first load item in config.sys to allocate the memoryspace between 512 and 640 for its window to take effect before the drivers for the non DMA devices in those times.