I have to agree with him, and I use Python on a daily basis. When using Python for anything larger than a few thousand lines of code, it really becomes necessary to switch to something more, well, stable. There's several options, and Java is one of them. When you start typing assert statements everywhere, that kinda defeats the purpose of using a dynamically typed language, and tells you it's time to switch to something more robust to begin with.
That doesn't mean Python is a bad language. I, in fact, love it. But it's not suitable for every use case. Just like Java ain't. Using the correct tools for the job is a better paradigm than sticking to your favorite language just because.