Earlier quoted context omitted.
> You should still have tests, but IMO tests that are just checking that a string is a string are The correct completion to this sentence is "irrelevant.", because that's not what anyone is proposing. The fact is, behavioral tests catch a lot of type errors even without intending to, and more to the point, if you test all the behavior you care about, then you don't care if there are type errors, because they only occ…
My personal real life experience with Ruby developers disagrees with you, but I accept that I could just have experienced a set of developers that weren't very good at testing.
It's impossible to defeat the claim that "with good tests you don't need static type checking", since every counter-example can be dismissed by arguing that better developers would have covered that test case.
I can claim just the same that "with correct code you don't need tests", and dismiss every counter-example by arguing that better developers wouldn't have made that mistake.
Obviously we know that developers sometimes make mistakes, and sometimes these mistakes are in the tests themselves. So trust your experience regarding how this all works out.