Why C# is not a good choice for web development?

Only if you don't make a real bugfix release, but throw the bugfix together with whatever you happen to have on your development machine.

Oh, and your "but in Python I can change a single file" leads to the exact problem, if you have continued development on that file.

That's all so incredibly stupid and irresponsible that I'd be inclined to say that OP's company doesn't have a problem with C#, but with its total lack of competence and responsibility in general.

What's frightening me even more is the number of "kudos" to that medium post.

Deployment process is never a good reason for choosing a language. Deployment can (must!) be automated and reproducible. Language choice is about hiring, development speed, rate of mistakes, and hardware cost for sufficient capacity.

The "problems" he talks about are caused by so many other problems he has (no version control, not knowing what's in production, making breaking changes as part of a small bug fix, not testing changes, etc, etc).

And, his proposed fix - use a scripting language - would break too if he made a backwards-incompatible change.