Well, I've seen it first hand on several sleeved blocks. I don't know what to tell you. I wouldn't have said it had I not experienced it. Two of the blocks were aluminum with factory iron sleeves, so they were done at the factory. I've even sonic tested two Ford 8N blocks for the heck of it. They have drop in sleeves.
Like I said in the post, if there is a different metal, iron sleeve in an aluminum block, and the tester was calibrated to iron, it would report the iron sleeve thickness.
If both the sleeve and the block were cast iron, there would need to be a gap between the sleeve and the block bore for the sonic testor to only see the sleeve. And if it was a drop in sleeve, there would be a gap, so yes I agree with you on that.
