To me it is a matter of effectiveness of resources. So i guess you could say the answer is yes, they target toxic chat because it is easier.
So you can spend X amount of resources on either trolling or chat. Focusing that on chat improves the player experience by Y. Focusing that on trolls you can improve the experience by Z. If Y is greater than Z then you get more out of cleaning up chat than you do cleaning up trolls.
Realistically though there is a lot of trolling that will probably never be caught. They will always have plausible deniability on their side. Most trolling is just doing normally inappropriate things at inappropriate times. Determining that they were doing it at that inappropriate time for the purpose of griefing is incredibly difficult, if not impossible in some cases. I don't think it is much of a matter of resources at this point with trolling. It is simply a matter of many cases being undeterminable.
It's kinda moot at this point anyway. While I'm sure there is still work on the automated chat system, it's automated and running. It's doing its thing without intervention. The automated system policing chat doesn't take much of anything away from anti trolling efforts at this point.
Trolls are rare. If riot waved a magic wand and made all the trolls disappear nothing about my experience would really change. I'm pretty sure I can count on 1 hand the number of trolls I have encountered in the thousands of games I have played since season 1.