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I have a Crucial 500 GB SSD (CT500MX500SSD4) which is reporting a non-zero number of data address mark errors. Other parameters appear fine (wear leveling is occurring, there are currently no I/O errors or failures at the OS or user-mode level, and the only other errors are old ultraDMA CRC errors that I have already ruled out as an old bad connection. The data address mark errors are new (they arose in the last two weeks, on a laptop with use of about 5-6 hours per day). All files can be successfully read (backups, including a recent full one, are succeeding).

I know that for spinning hard disks, this parameter is informational and does not indicate imminent drive failure (source), because this error may arise through benign one-time servomechanism seek/addressing failures or local degradation. However, SSDs locate blocks by addressing directly into flash chips without any opportunity for electromechanical error.

Does this mean that data address mark errors, on an SSD in particular, are a sign that indicates imminent or ongoing drive failure?

In this HD Sentinel post
it is answered:

In general, the manufacturer for most SSD devices define a method to measure the overall "health" of the memory cells in the SSD.
Usually there is a special attribute (for example, #202 Percentage Of The Rated Lifetime Used in your SSD) shows the calculated value, in this case 50%.

Further, the article
Predicting SSD Failures: Specific S.M.A.R.T. Values
says:

Percentage of the Rated Lifetime Used. This is the opposite of SSD Time Left. 1 means the drive is 100% healthy, while 100 means that 100% of the drive’s lifetime is used up, and the drive can be used as a small doorstop.

This seems then like an indicator which might indicate when an SSD is nearing
its end of life, according to the manufacturer.
It doesn't mean that the drive will immediately fail when it reaches 100%,
but that the drive shouldn't be relied upon to continue on working
without problem.