A suit of mithral armor that fits a Medium creature
The wording is indeed confusing, but the key is in the capitalization. Creature size is always capitalized. For example, the description of oil of etherealness says:
The oil can cover a Medium or smaller creature [...] (one additional vial is required for each size category above Medium).
"Isle of the Abbey" isn't the first place in Ghosts of Saltmarsh where armor sizing is mentioned. In "The Sinister Secret of Saltmarsh", the party can come across a dead adventure wearing plate (GoS, p. 45):
The plate mail is banged up - the result of brutal clubbing and kicking - but serviceable and fits a Medium creature.
Again, creature size is capitalized. This also sets a precedent - armor will include sizing in Ghosts of Saltmarsh, likely a hold-over from the early editions that many of the adventures were written for.
On the other hand, armor categories are not capitalized. From the header of mithral armor itself:
Armor (medium or heavy, but not hide), uncommon
The barbarian's Rage feature also mentions armor by category (again, without capitalization):
While raging, you gain the following benefits if you aren't wearing heavy armor
Lastly, the latest version of the AL Content Catalog (v8.09) does not contain specific guidance about this treasure. It does include guidance about mithral armor (plate) that appears in a later adventure, so either they missed this one, or they did not intend for it to be changed. For now, at least, the DM can give out any medium or heavy mithral armor.