This belongs in the Mac Mini forum, but I can still answer the question 😉
HDD can be (albeit incorrectly) used as an umbrella term for all fixed disk drives (so both spinning platters and SSDs). Likely what they meant was that it came with a hard drive installed. This would be important to note because not all used computers come with one, as it's often easier to remove a hard drive from a computer rather than to wipe it.