that's a bit of a stretch. hank needs a warrant to tap walter's phone, especially if he wants that evidence to be admissible in court. and so far walt think's he's got hank against the ropes with that video he made.
That's just sloppy no matter how you slice it, though. Even the peon 13-year-olds in the hood on
The Wire know you don't do anything over the phone that will incriminate you.
Hell, he knows Hank is slipping GPS monitors under the wheel well of his car already.
I doubt what he said would be admissible in court as you said -- which I think the writers conveniently hid behind when they put Walt in cuffs. But admissible or not, the more you divulge the more you give the DEA ammunition to know where to look and what to presume. So it's silly that Walt suddenly became so uncharacteristically careless and sloppy after everything he's done to date.