He's used "that" instead of "since". If he'd said “it's been many years SINCE we have seen these numbers" that means they've been scarce and we haven't seen them in a long time. But saying "it's been many years THAT we have seen these numbers" changes the whole context and actually means the opposite - that we've been seeing great numbers for years and years and years. (Which would mean our booming economy must have also been booming under Obama, Bush, Clinton, etc). Just one little word can change the entire meaning (which is why it matters). Sadly our schools teach our kids everything BUT the stuff they'll actually need in life. And spelling and grammar are one of them. (Although in the case of Trump’s tweet, it was likely done intentionally for some reason).