The Moon is a world and it offers several advantages compared to a space station: It can protect astronauts from micrometeorites, solar radiation, cosmic rays (the moon used as shelter). It can keep the habitat at a constant temperature and especially: it provides astronauts with a gravity and a place to study and we don't yet know everything that's out there. Practicing for Mars means learning how to use, reuse for months at a time, the spacesuits, the mobile vehicles, learning how to cope with lower gravity. And learning how to recognize brecchias from anorthosite or ancient lava.
Practicing for Mars does not mean to launch from the Moon (which he doesn't even imply) to Mars.
He also points to the cosmonauts' experience in very long stays in the stations. They've experienced leaks, fires, collisions, ripped spacesuits, toothache, prostate inflammation, boredom, continuous failures, crew misunderstandings. But they also experienced repairs and learning on the spot how to fix things even without having trained for them.
The Russians are, so far, much better trained for a Mars flight than the US astronauts. The reason is that in many occasions, the Russans could only rely on themselves and there was no money nor time for a replacement.

Raoul