Don't the old street fighters handle it that way?
No sir/madam. Let's explain:
A meaty attack in any game allows you to LINK to moves with slower startup than would normally link afterward, because a link requires you to go through the entire animation of the move. So if the move has 5 active frames and you hit with the last one, that gives the total move animation 4f less recovery after the hit.
A meaty attack does not allow you to CANCEL and combo into moves with slower startup than would normally combo, because the cancel occurs on the frame after it is input. So if the move has 5f active time, and you cancel immediately, it does not matter whether you hit with the first active frame or the last, what combos afterward is still the same.
As far as the explanation posted on Twitter goes:
In SFV ONLY, because I am not aware of any other fighting game in existence that works like this, if a move has 5f active time, and you input a special cancel immediately when it hits, the cancel will not occur until after all 5 of those potentially-active frames have passed. So if you hit with the first active frame, the cancel will not occur until 4f later. But if you hit with the last active frame, the cancel will occur immediately, which effectively gives you 4f more hitstun for the followup to combo.
Related to this, changing how long a move is active for would then potentially change what moves will combo after it when cancelled into, even if the actual hitstun of the move is unchanged. This is pretty weird.