A reminder: economy is not hard science. Even if it was hard science you’d still have room for contingency, because universe. Also you cannot evaluate future events in economy, because future.
But let’s step back from epistemology to politics: Could he give a number without lying? Yes, upon one condition: he must promise. If he fails his promise, he fails because universe + future. What’s the political repercussion of that? Will he be disqualified from presidency if he meets %3? No. Will he lose an election? Maybe. If his economic policies fail, will he lose the next election? Probably. Would it matter that he failed his promise by a large margin? No.
So what’s your real political point? If you don’t have any why are you voting for a Bernie with an ‘of course’?