In core 5e RAW rules, Javelins are simple melee weapons (STR based). However in this particular players guide they talk about Ranged weapons, which Javelins are mentioned.
On Page 20 of the "Odyssey of the Dragon lord" players manual:
Thylean Weapons Warriors in Thylea typically use spears, shields, and swords. There are exceptions, of course, but the armies of Mytros and Aresia train thousands of soldiers in the use of these simple armaments. Heavier weapons, such as halberds and greataxes, are thought to be barbaric.
Ranged weapons are typically limited to javelins, slings, longbows, and shortbows, although there are some who have adopted the use of complex recent inventions, such as crossbows. These weapons are engineered in small quantities by the Academy and are not widely available. Thylea boasts a handful of unique cultural weapons, which can be found at just about any blacksmith or adventuring shop. With the exception of the chakram, they function identically to their normal counterparts (unless they are magical).
There are other places where they change/add new descriptors to existing core weapons. For example page 49 of the same players guide:
Vagrant Soldier Despite your roguish demeanor, you have all the training of a common soldier. When you choose this archetype at 3rd level, you gain proficiency with shields. Additionally, spears and tridents count as finesse weapons for you.
If anyone has played this campaign and can prove a definitive ruling on this question – using RAW or other items from the publishers, that would be great.
As usually, it is up to the DMs discretion for a final ruling – this is a known and doesn’t need further mentioning as part of an answer.