I continue to get the vibe that this whole "allow witnesses" brouhaha is really a "briar batch" moment for the Trump team. They'd rather just wrap this impeachment up quickly, but if the Dems successfully pull the witnesses lever, with help from classic unwitting dupe Romney and others, Trump's team is going to unleash holy hell on them all.
And Bolton's going to be completely unworthy of the efforts that the Dems are expending to hear from him. He's got nothing earth-shattering to add. He's being coy because he's selling books and he likes screwing with them.