It's a warning to others about asp and Windows and not that he rewrite everything even though I would (and once did).
Mono is for people who have Windows code they wrote on and for Windows and now they want it to run on *nix. If you are stuck with that then go ahead but Mono is always far behind .NET versions and it's still Windows code trying to run on *nix. The natural environment for the web is *nix and the tools there are all geared toward networking and the web so it's always best to be where the hand fits the glove.
I first started my web dev company 9 years ago with .NET. I will never make that mistake again. While .NET changed/upgraded/deleted/charged me over that short time, I am still running all the same *nix/BSD code from then and for free.