As you're from Germany you're able to read the whole text and understand what's written their. The identified two major problems:
a) iPhone is not optimized to the network of the provider
b) Design of the the iPhone ist not the best for perfect reception of mobile signal
ad a) T-Mobile is the worst carrier in Austria. Some say Orange but Orange has not the best coverage but a stable network. T-Mobile offers in wide areas of Austria only mobile speeds up to 1MB Down und if you're lucky 1MB Up. On the same place with Hutchsion 3 or A1 you'll get most likely 20MB Down and 4-5MB Up. T-Mobile invested in upgrading their network to SingleRAN and this project finished in Oct/Nov 2012 to be able to offer LTE if new frequencies are available after an auction. Befor the whole network was not updated since years! There have been a few LTE field tests but limited to small areas (Vienna/Graz ... and don't know from more ...).
Also 3 has issues with the iPhone as therei AMR-WB implementation is screwed up and does not work with the iPhone5. I told this multiple times in this thread.
To be fair, this test says that T-Mobile has the best voice quality (in terms of MOS-LQO/MOS-LQO) and success connections. BUT the test was critized because only main spot areas were tested. So let's make a reality check. Austria is a land rich of mountains and valleys. So primary the test is just for marketing as it HUGELY depends on where you are in Austria. The best carrier sometimes doesn't have coverage or only GSM where 3 Hutchison has HDSPA+ Coverage. Overall the test states that every single carrier in Austria is still better in terms of quality than any other carrier in Germany.
ad b) iPhone5 had one of the best receptions i've ever sawn. The made great improvements and learned from Antenna-Gate. Personally i had EDGE coverage in elevators with my iPhone5 ...
There's one thing the article didn't mentioned. Did the measured just iPhone 5 from their own network? T-Mobile is a fallback network for 3 Hutchison. Where no 3G coverage is available you automatically roam (not multiple operator, single network technology) into the network of T-Mobile. So maybe the whole article is distorted because of issues while roaming back to the 3G network OR not roaming back as it never was possible for me during a call ...). But this is one major source of issues!
And as an additional information -> there's an ongoing discussion on a quite huge website in Austria about this article and most people aren't able to comprehend this article because personal experience states the oponnent.