Headset labeling - Does this qualify as EC30? I am currently in a dispute with an eBay vendor about the sizing of a threaded headset I purchased. Before I initiate a refund claim, I would like to make sure I'm in the right.

The headset's manufacturer is XLC and the model is HS-S01. The eBay listing describes its dimensions as "1" EC30/22.2 EC30/26.4". When installing it on my bike frame, the cups went into the headset bore without requiring any press-fitting, and they are so lose that they slide right off when not held in place.

The frame has a headset bore of 30.1mm. According to my research from the Park Tool documentation and other sources, this is compliant with the EC30 SHIS standard, and a headset advertised as EC30 should achieve proper press-fit.

There is actually no mention of a SHIS code on the part label, which simply describes the item's dimensions as 22.2/30.0/26.4. It's unclear if the 30.0mm figure refers to the frame bore diameter, as the manufacturer's website seems to indicate (see the link to XLC above), or the cup diameter. On the part I got, I measured a 30.03mm cup diameter.

My argument is that the vendor's mislabeled the item by adding the EC30 SHIS code, which specifies a bore diameter that is incompatible with the part, regardless of how you interpret the manufacturer's labeling. If anything, this is an EC29 part, which would fit the actual cup size.

Based on the information above, is that argument correct?
30.03mm is a measurement one might expect to get off the OD of a 1" JIS cup (called EC29 in SHIS, from the 29.8mm frame bore). That's consistent with the "22.2/30.0/26.4" on the label.

However, 26.4 is the ID of a typical crown race from a 1" ISO/Campy/Professional headset, aka EC30. 

You are in possession of a mixed standards headset. These have always been around because some bikes actually come with a mixed standard on their parts, for reasons I can only speculate on. In SHIS parlance this would be EC29/26. The cup size plus a forward slash followed by the SHIS name for the crown race size is how this is communicated in SHIS.

In pre-SHIS days (SHIS is young in the history of bikes), the "22.2/30.0/26.4" label would be the piece of information that communicates this mixed standards configuration. It is naming the physical sizes of the parts, not the frame fitment. It's kinda implicit that this is the case when you look at it, but for a tricky reason: the way the numbers line up, as you've noticed, "30.0" could be taken as either the OD of a JIS cup or the ID of an ISO headtube. This has always thrown people for a loop. But 26.4 is only the ID of an ISO race, as a JIS race is the very different 27.0, so it should be assumed that the label is stating the dimensions of the parts inside, not the frame/fork dimensions.

Mixed standard headsets are pretty obscure, so this is an understandable mistake on the seller's part.