Thanks for working on this

Once upon a time, when I was a PhD student, I remember this person... When I (or some other student) sent him a report, or a draft of document, or something similar, he used to start his reply with "Thanks for working on this!".
After this good start, he basically destroyed every single part of the document by (rightfully) criticizing a lot of aspects, etc...
So, his emails looked like this:

Hi Luca;
thanks for working on this!
And, BTW, this is wrong ... that should be explained in a different way ... this you did not understand... Please rewrite that...

and so on... And the frustrating thing is that he was right 99% of the time. I always suspected that he used the "thanks for working on this" thing for making all his critics more acceptable.

Now, the scary thing is that few days ago I found myself writing "thanks for working on this" to someone... Does this mean that I crossed the line?