When in Rome (last about 2 years ago, next will be this coming Sept), we spend a bit of our time in Testaccio. We eat at Flavio al Velavevodetto & Trat. Perilli, peruse Volpetti and love wandering the back streets of this neighborhood. The area used to have a great old outdoor (but covered) market, a couple of blocks behind Via Marmorata. Around 5 years ago (my years tend to run together these days, but I think that's about right), due to neighborhood "development", they moved the market to where you found it. When they did, they only set up a small piece of it and continued the process of building new sections that you've now found fully operational. Looks like they did a major increase in space since we were there and it does look a lot like the similar renovation of the old Florence Central Market that was also completed in the past 2 years (the upstairs portion of that one was closed for several years).
At any rate, I (again) feel old bemoaning how things "used to be" but we really loved the old Testaccio market. It was a very old world place, with several vendors who were there for a very long time and were feeling squeezed out of their own area by the gentrification. Life in the big city, much like home in Brooklyn. When we went to the beginnings of the new place 2 years ago, the market was better organized, more businesslike and, to us, lacked the charm of the old place. Needless to say, there was a lot of opinion pro & con about these changes, but it seems that acceptance has come. As usual, Katie Parla has a good piece on it http://katieparla.co...staccio-market/ that you might like to read.
All in all, glad you found our favorite neighborhood. And even more glad when you post about your adventures. They're some of the best things I read here. Thanks.