Your mechanic may be right, or at least think he is. For Engine code CCTA, there are two converters - one in the space of that "lump" in the left side of your picture, and the so-called "center" converter. The primary converter probably isn't large enough to handle the whole cleanup on the turbo CCTA engine.
That may not be the right picture for your vehicle. Below is the exhaust system for the CCTA engine, showing the O2 sensor bungs right at the turbo flange, and the downstream sensor (wide range six wire) after the center converter.
Alldata even depicts some variants with three O2 sensors on the ECU, but I could not find a physical picture to prove it.
Check around the turbo flange to see if there's an O2 sensor there. In that case, I'm pretty sure there's a catalyst honeycomb in the barrel "lump" after. (But SolarMike is quite correct; the item in your green box is merely a flexible joint.)
In any case, P0420 comes from information from the last sensor, or the one located after the center catalyst. FWIW... There's really no telling which single converter is causing the problem, or maybe both, or maybe the P0420 is caused by 100 other things that seem to indite the converter(s), but are actually due to A/F ratio issues that deplete the oxygen storage. It might even be due to a failing O2 sensor.
If you are certain you have the O2 sensors only in the locations you describe, you may have the CBFA engine varient, just one converter, and your "yelp quote" mechanic hasn't done as much homework as you have. I suspect the latter.