Waterfall is just a strawman?

Is waterfall just a strawman used by Agile evangelists?

"strawman" implies that waterfall is not a real problem, that it no longer exists in the modern world of software development.

I don't really know how to respond to this except to suggest to look a little closer at how most IT organisations actually work.

But perhaps it's just a problem for "enterprises" and not "startups" and Silicon Valley?

There are two extremes that lightweight methodologies were a counter for:

1. Waterfall
2. Code and fix

Enterprises tend to have the first problem; startups tend to have the latter.

Neither waterfall nor "code and fix" are strawmen within enterprises as they typically have examples of both.

Waterfall is not a strawman but is probably not the problem you're facing as a startup.  That problem is either code and fix and/or the so-called "engineering-driven company" which is why Lean Startup was invented.