This is the first time I've seen a self-study with l-theanine in the context of stress relief. In fact, somehow, it's the first time I've ever seen stress relief mentioned on the topic. For me it has always been about providing a small cognitive boost. The result is clear: for this particular individual, and the specific goal of stress relief, l-theanine does not work.
My anecdote: I used l-theanine supplement for about a year straight. To me, the noticable effect was similar to that of a cup of coffee but with practically none of the jitters.