This is utter bull, even if PG said it. I've learned a huge amount about Java by using an editor (Intellij) that has real time static analysis of my code and points out when I'm making common mistakes with corners of the language (calling overrideable methods during object construction, for example). It's incredibly useful and informative.
Perhaps out of date now, given what IDE's can do, though I personally still prefer to learn a language and toolchain with a text editor, then graduate to Eclipse for the benefits you mention.
