N Korea can NOT flatten Seoul in a few moments' notice because of the following:
1) Seoul is a HUGE city.
2) N Korea actually doesn't have UNlimited number of artillery tubes capable of reaching Seoul. There are hundreds (maybe a few thousand) tubes but these are heavy ones, meaning they would be lucky to get off a shot every few minutes. And they would have to be concentrated in a relatively small area near DMZ to reach Seoul, not smart move considering SKorea/US's jets.
3) You can BEN SKorea/US jets/artillery are already targeted for some/many of the known N Korean artillery sites.
4) N Korea's most interested in preserving status quo for their leaders and the elites that support the top leaders. Flattening Seoul would certainly mean they would be eliminated.
Yes N Korea can damage Seoul, but will not really 'flatten' it. Well, I hope...
There are some very detailed writeups of PRNK's ability to damage Seoul based on open sources around on the web. The short summary is that 'flatten in a few moments' may be an exaggeration, but they do present a very real and very large threat.
They do have a fair number of dug in artillery installations that could survive several airstrikes. The basic template of these installations are bunkers under mountaintops that artillery can move rapidly in and out of. So instead of shoot and scoot more like shoot and turtle up. They may have limited resources, but the PRNK does appear to know how to deploy them quite effectively. Chemical shelling is a particular concern.
It's not that the PRNK would win an outright war, it's that they could extract a massacre no one else wants.
The op was referring to "flatten" in terms of using a nuclear weapon not conventional artillery. Regardless, I think either way is so improbable it probably isn't even worth speculating about.
Yes N Korea can damage Seoul, but will not really 'flatten' it. Well, I hope...