Netscape had javascript running server-side in 1995, only a couple years after Lua was created. So no, Lua did not have "over a decade head start", it had 1 year.

Microsoft also had server-side JS, and still do. Active Server Pages (ASP) were scriptable with javascript since 1996. Microsoft's JScript.net has been around since 2002 (even if not well supported or maintained by M$, it still works). I've written web back-ends, Windows desktop applications, and even .dlls with JScript.net as far back as 2003.

Javascript has been Adobe's scripting engine for Photoshop since 2005. Sony chose Javascript for Vegas Video's scripting language in 2003.

Javascript has so many uses outside of a web browser, and has for a very long time.

Yes but for the first 10 years (1995 - ~2005) JavaScript was not taken seriously as an application language in most dev shops. Active Server Pages were overwhelmingly coded in VBScript. JavaScript was used in little snippets in "onBlur" attributes to do client-side validation of fields on a form, pop up "alert" boxes, and enable submit buttons. It wasn't until XMLHttp and prototype.js and jquery and stuff like that came along that mainstream developers started to understand what JavaScript was capable of.

I guess I wasn't a "mainstream" developer in the 1990's? Before XMLHTTPRequest, I was using iframes to asynchronously load content into the browser, essentially accomplishing the same thing before "AJAX" was a thing. We were doing long-polling before there was websockets. I was using Javascript and CSS to create "DHTML" SPAs before XMLHTTPRequest, with Javascript for front-end and back-end. You either know the potential of the tools that are available, or you don't.

But I guess my anecdotal experience doesn't matter, because I wasn't a "mainstream" developer? From my point of view, everyone else was missing out on using one language for front-end and back-end.

Context switching has a real cost in terms of developer burnout, and it's something I've happily avoided for 25 years.