In addition to the other great answers, I feel like I should bring up a relatively new phenomena on the internet. For the purposes of an easy-to-understand explanation, I'll use a frequent example between my friends and I.
Me: Here's a link to an adorable dog gif!
Them: Thank you
In this situation, 'Thank you' is intentionally written as overly-serious. This creates a humorous contrast between the light-hearted, off-handed nature of the link provided and the very professional response. The irony is appreciated by both parties, frequently causing both of us to chuckle.
This type of scene is rising in frequency on certain social media websites, so if this new friend of yours frequents things like Reddit or Imgur, it's possible this is what he's doing.
I'm unsure as to the origin of this, but it's possible that it shares some similarities with the Me Too Thanks Meme. Both of the memes derive at least part of their humor from the unexpected nature of the reply. For example:
Two bees are talking to each other
theres 20 letters in the alphabet right
thats totally wrong there are 26
o yea I guess I forgot the letters; u r a b
thats funny b/c i am a bee
me too thanks
me too thanks
This example is similar to an abrupt 'Thank you' due to the sudden, unexpected nature of the response.
Without further context or understanding of the personality-type of this individual, I cannot confirm nor deny whether this interaction matches my example.