this is because it is implied that watching the video has already occurred. So: I watched the video already and now i have earned a reward". If it was assumed that the watching has not yet occurred you would say: "after watching this video i will have earned a cup of tea."

It is not describing an upcoming event. The earning has been done before making this statement, and the use of present perfect shows that it is a present status resulting from a recent action. 'After this video' is a prepositional phrase which gives this sentence a chronological order along with the use of present perfect.

When you us the word "after" you are NOT making a specific time reference. Therefore, using simple past or present perfect can BOTH be OK depending on what you are trying to communicate. After this video, I've earned a nice cup of tea. ✓ After this video, I earned a nice cup of tea. ✓ Both of these sentences are perfectly OK. Using a phrase such as "after..." does NOT put a restriction on what aspect (simple vs. perfect) the second clause needs to be presented in.