You’re arguing that the guarantee he’s making that the bill will be passed into law - he would be lying if so, for exactly the reasons you stated above, but you’re mistaken in what he’s actually saying.

The guarantee being made is in terms of this bill’s ability to achieve what he’s claiming it can.

Hence ‘I can because I wrote the damn thing’ as his retort - he’s guaranteeing the claims promised within this bill to be what he knows - as its author - has the framework, planning and effectiveness to achieve the goals set out within. He’s guaranteeing that what he’s been successful in devising is a workable solution for delivering what he’s promising via this bill and that this is his guarantee.

In other words, you’re seeing this being Bernie guaranteeing the outcome - promising it’ll happen should he get elected, which is something he has no way of guaranteeing because it won’t be his choice to make.

But the guarantee he actually makes here is that he’s willing to stand over the voracity of the bill’s framework and guarantee that it can really do what he says it can. Which is a powerful statement to make and one he will likely have to be able to justify should this been questioned as the race goes on.

This make any sense?

TL:DR - he’s not guaranteeing the bill will be passed - he’s guaranteeing it’ll deliver on its claims were that ever to happen