Of course, StringUtils is not in the core API; the first idiom "foo".equals(bar) works in Java everywhere, StringUtils.equals("foo", bar) will only work in projects that pull in org.apache.commons.
Personally I prefer using facilities in the core API over third party APIs, even if the third party API is included anyway for some essential function.
I've been coding in Java for over 10 years. If I ran across pretty much any project that wasn't including Apache commons I'd be startled. I get your point, but some libraries are in such common use that for all practical purposes I consider them part of the language API.