1. Stealth for fighters isn't about keeping the enemy from seeing you entirely. That went away a long time ago. Stealth helps your survivability by making it take a little longer for you to be seen. If you can see the enemy from 50 miles away, but they can't see you until 20 miles away then you have a huge advantage in being able to launch a missile first.
2. Stealth is not just about RF, it's about visual and thermal cues too. The latest generation of missiles are going to multi-mode seekers (RF + IR is one possibility) so you need stealth in multiple domains to be effective.
3. RF stealth can only be optimized for one band. So you might design your coatings and structures to work against X-band (10GHz-ish) but be effectively 4th-gen non-stealth against early warning recievers that operate in the MHz range and against short range radar in the Ka-band (40 GHz-ish). This is why point 1 has become the way it is. The enemy will know you're there.
4. China is great at some areas, and laughably bad at others. They haven't put the WS-15 engine on the J-20 yet because they don't know how to make it right. They are currently spending tons of money on researching metallurgy so they can catch up with the rest of the developed world. It's funny that they can develop their own advanced radar and jammers but they still have to buy all their engines from Russia.
5. Good missiles are a huge component in survivability- a long range missile with lock on after launch capability can be fired in "the general direction" of a target without you having to have your own lock on them. This is good if you have an X-band radar on your jet but can get cues from a surveillance radar in another band, and your missile seeker is also in another band. This means your weak point that the enemy stealth was optimized for is covered by the strengths of your other air power. China makes great missiles.
6. Command, Control, and Communication (and Cyber, Intelligence, Surveillance, and Reconnaissance) play a huge part in a major air battle. It's not just a bunch of individuals trying to get their guy, but a coordinated strategy. China is especially dependent on control and communication, since their pilots are trained to follow orders from the ground. You need jam-resistant communications for this. China is great with RF stuff.
7. The article claims jets like the J-11 are 3rd gen equivalent and J-20 are 4th gen equivalent. This is almost surely an intentional downgrade because if this D.C. based think tank revealed what they actually thought it might hint that they know more about these jets than they want to. Calling a J-11 a 3rd gen is laughable. The latest J-11B is basically 4.5 gen. It can match or even outperform a fully upgraded F-15C in many areas. While it may not be better overall than a F-15C, it is at least 4th gen, call it 4.25 gen to a new F-15C's 4.5 gen. The same goes for the J-20, it is clearly better than 4th gen, call it 4.9 gen to the F-22's 5th gen.
8. While a J-20 might not match a F-22 one-on-one, if they can make the J-20 en masse, they will present a serious threat to the F-22. We only built 187 F-22s, haven't fully upgraded all of them (some don't need upgrading since they are only used for training), have crashed a few, and cannabalized others for parts. We'd be lucky to get 100 fully upgraded F-22s to a fight. We're hoping that we can make enough F-35s to cover that small number, but China is making the J-31 to counter.
In conclusion- Does China's J-20 rival other stealth fighters? Yes. Maybe they aren't fully competing in the stealth category, but you should be afraid of it because it has other strengths.
Edit: Danm, I almost forgot about electronic warfare (EW). Another alternative to stealth is jamming the other guy's ears off. It makes you vulnerable to anti-radiation missiles, but missile launch detection and a good on/off switch makes jamming a great self-protection scheme. Coherent distributed jamming is on the horizon, possibly to the point of fooling anti radiation missiles. China's pretty good at EW, just like they are at radar and communication stuff. The next all out war where developed countries throw all of their technology at each other is going to suck.