Is WTFast against ToS?

[Shioban]

a parser does not affect data in any way, shape, form, or fassion. it reads what is posted on a chat log and compiles it into an easy to read format. however, a vpn does alter the route the data is sent from your isp to the server so it does DIRECTLY alter data.

Parsers directly read the battle logs and memory dumps from FFXIV battle encounters which is seen by the development team as an unfair advantage as not all players have access to it.
So yes, it doesn't alter the data but it is reading it, which is against their terms of service against third party applications.

(Fixing your awful internet connection through any of these 3rd party programmes isn't an unfair advantage)

Arguing otherwise is silly.