What evidence did he cite?
Kaku discussed the emergence of intelligent life on Earth, noting that the dinos seem to never have had any during their 200 million years, and Tabby's Star dimming, and the exoplanet count (5000), and that 20 Earthlike ones are known.
Thousands more candidates have been identified but not yet verified (it will take a while, obviously, due to observatory capacity).
NASA's exoplanet page: "Since the first exoplanets were discovered in the early 1990s, the number of known exoplanets has doubled approximately every 27 months."
The problem here is not observational, it's psychological -- 'we can't get there, so no reason to look'; 'conditions on Earth were perfect (not until 200,000 years ago though) and WON'T be found ANYWHERE ELSE no matter HOW HARD YOU LOOK',; etc.