I found a different translation from the one that you put in your post, that might help you understand the meaning of this sentence:

I remember this image: of being a child, a teenager, when Paris was different. This was before the War. The summers were hotter, certainly hotter than now as everyone believes, or at least, me, I do believe it. Very often, the Parisian concierges, at the time an institution, came out with their chairs during the evenings, propping them in front of their doors to sit without doing anything.

This is, of course, a vision of laziness that has been completely eliminated, and which I no longer find in contemporary life; in today’s Paris, there aren’t many gestures of laziness. A café is nonetheless a relay point with conversations and an “appearance” as well. This is not true idleness.

If I take Google’s definition for "spin-off":

A by-product or incidental result of a larger project.

I think he is trying to say that when you go to a café, you get something in return (a coffee, a drink, conversations with people). A result is produced, it involves the appearance of an activity, and therefore it is not an example of true idleness (in opposition to the concierges simply sitting down in front of their doors).

I hope it makes it clearer. As a French native, I tried to find the French version of the article to understand it better, but I only found an incomplete one that omits this very part of the interview.