Aside from the obvious inaccuracies in that CEO's post (.Net is not a language, it's a managed runtime, analogous to the JVM), he's obviously not spent much time with .Net, and his knowledge of it sounds limited to what you'd find in an advertisement for Visual Studio about 7 years ago. It's about as accurate as saying PHP is the best language to build a web startup because so many sites on the web use it. (Yeah, please stop that, I don't want to clean anymore of that up. I'm going to self-select against startups that choose PHP because only about 1 in 50 PHP shops have code that's even worth trying to maintain).
I've spent time with just about every mainstream language and platform, a fair number of not-so-mainstream ones, and cut my teeth on Basic, Logo, and Assembly language programming on a TI-99/4A and the Commodore 64/128. The reason I have done this is because I like learning these things.
The one valid, useful point that that CEO makes in his rant is that developers like to develop. I chose to learn .Net partly because I worked for Microsoft in the late 90s, but also because I saw some value in it and thought it might improve my productivity. It's now a much better ecosystem than it was when it was widely perceived as a tepid response to Java; C# and F# are far more expressive and productive as languages than Java appears to ever hope to be, and frameworks like Asp.Net MVC make up for most of the frustrations I ever had with the old-school VB forms inspired WebForms feature of Asp.Net.
There are some cultural issues that I've encountered in the .Net world; a number of long-time Microsoft-stack developers were relatively slow to warm to ORMs, dependency injection, TDD, loose coupling, and similar techniques even though long-time, syntactically frustrated Java developers had presumed these things were just What You Do (TM). But I've encountered crap code in every possible technology stack.
Given my druthers, I'd definitely pick a Rails shop over an Asp.Net Webforms shop, but it's pretty much a wash if I were choosing between Rails and Asp.Net MVC and FluentNhibernate and all the tools money can buy. On the other hand, I will never again choose to work in a shop that decides to build out their architecture in Perl or PHP, unless that Perl is written by Brian D Foy, or the developers at least had the good sense to build their PHP app in a decent MVC framework, and everything else about the company rubs me the right way. (Full disclosure: I am, in fact, working for a company that built out their architecture in Perl and PHP. Ugh. But we're trying to fix that.). Life's short; I want to program in something that brings me joy.
But, most importantly, I'd never go to work for a CEO who thinks that because you've learned something, you're less valuable than someone who hasn't learned it.