This occurs through various calculation schemes linked to child-based deductions, a removal of the marriage penalty and several other programs. This plan that funnels most of the money to higher income earners is still called progressive because the lower income earners get more tax relief in percentage terms. Plans that address 'inequities in the tax code' will always have this feature. Any desire to funnel more dollars to lower income people will have to adjust the marginal rates even lower or propose actual credits or payments and deductions that come with 'income tests'. This effect remains one over which there is much in dispute.
Lock boxes are the stuff of rhetoric and image.
To keep the metaphor, lock boxes have keys and locks can be picked. Congress can undo whatever it does. That is good news as labor markets stay tight and as energy prices continue to ratchet up. But this is something to watch carefully. This is still speculative.