No, what it means is it can go both ways. Lol There are 3 tiers to melee, and soon-to-be 4 tiers to guns. Unless another tier of melee is introduced, there will always be this confusion.
Allow me to break it down for ya.
A18
No official T0. Thus, back then, the wooden club was a T1 melee weapon much like the double barrel shotgun was the T1 shotgun. (Blunderbuss was technically T0, but it was an exception, and come on, very, very few people used the thing.) Therefore, Tiers 1-3 existed. (Example: Stone Spear (T1), Iron Spear (T2), and Steel Spear (T3); however, a lot of the firearm classes were incomplete since they were missing some T2 or T3 variants, which were added in A19. Agility was and still is an exception, sporting two T3 guns, and Intellect melee was and still is an exception, only including the Stun Baton.)
A19
The primitive age is added. Former T1 melee weapons, like the wooden club, are technically reclassified as T0, but it's not incorrect to identify them as their former labels either. The bone knife, for example, could be labeled either as T1 or as T0. Guns will have 4 tiers in A20, melee will still have 3.
To get all detailed about it, there are 3 tiers to melee, hence why people use the term "T1, T2, and T3", but with the primitive age added, they can now be reclassified as T0, T1, and T2. Basically, the term "tier" now has 2 different meanings: the age in which it is crafted/looted, and the number of melee weapons within the class itself.
Make sense?