While acknowledging India's definite conventional superiority over Pakistan, the argument is that it is not enough to "wipe out" Pakistan.
Enough to defeat and badly maul any invading Paki forces, yes. Enough to push into Pakistan and take over large-ish tracts of land, yes. But not "wipe out" or "put an end to" Pakistan, particularly after a nuke exchange.
Let me elaborate. Assume that if there were to be a nuke attack on India, at least half would be on military targets (rather than 100% on civilian targets). Also assume, worst case, that they have 100 usable nukes. So, we can expect:
(1) 25-30 nukes used for: massive damage to, or destruction of, key airfields: Ambala, Amritsar, Awantipur, Chandigarh, Halwara, Hindon, Pathankot, Sirsa, Baghdogra, Kalaikunda, Agra, Gorakhpur, Gwalior, Bhuj, Jaisalmer, Jodhpur, Jamnagar, Lohegaon, Bikaner, Phalodi, Yelhanka, Nagpur
(2) 8-10 nukes used for: hitting key navy installations and ports: Mumbai, Vishakapatnam (within missile range?), Karwar, Goa.
(3) 20 odd nukes used for: Hitting key army formations/commands: Southern Command Pune, XII Corps Jodhpur, XXI Corps Bhopal, Jhansi, SW Command Jaipur, I Corps Mathura, Ambala, Bhatinda, Sri Ganganagar, Kota, Bikaner, Patiala, Meerut, IX Corps at Yol, XI Corps Jalandhar, Firozpur, Amritsar
(Note I have not included any targets in J&K).
Even after this attack pattern, the Pakis would have another 40-odd nukes to hit political/civilian/infra/economic targets.
Now, the question is: if this were to happen, estimate the damage and destruction to our military infrastructure and assets. For example, what % of air assets would we be left with? And then see if our armed forces would be in a position to "wipe out" Pakistan.
We would of course hit them very hard, wipe out almost all their military assets, but not completely destroy Pakistan if by such destruction you mean occupy every inch of land or as some posters seem to suggest, kill every Pakistani. Perhaps India could still muster enough conventional forces to occupy Pakistani Punjab and Sindh - but to what end? Holding on to that territory as an occupier is not as easy as it sounds; look at the US in Afghanistan and Iraq.
I believe that is what Kannan was referring to - please note that in any case India would still retain conventional superiority over Pakistan and that is not being debated.