Well I'd liken it to a Terry Pratchett quote about witchcraft - you can teach yourself, but both the student and the teacher need to be the right person
Yes, people can teach themselves. It doesn't follow that all self-taught people will be successful at it (some will be extremely successful, some will be lousy, most will fall in between), and most self-taught people I know have picked up a bad habit or two and had to work hard to cure problems which a good teacher (and yes, sadly not all teachers are) would never have let become entrenched, and which in many cases could have been easily avoided
Even people with considerable musical experience can end up with a nasty bad habit that only becomes a problem at a later date (There are plenty of little niggly things that aren't obviously wrong unless you know the instrument, and that one can "get away with" for a while and then, bam, find it's impeding suddenly progress and becoming a real bugbear and hard work to fix)
So if people can afford to get lessons/can find a decent teacher - even if only occasional, or a starter course - from a good teacher to get them going in the right direction, it is (IMO) a Good Thing Self-teaching can be a lot of fun and very rewarding (looks who's talking - I have one teacher at the moment and that's on the instrument I have probably the least aptitude for... who am I kidding, the instrument I have NO aptitude for!) but it can also be risky. 6 months of lessons with a really good teacher can be a real investment that will set someone up for many years of playing.