The forearms are vertical with the elbows just in front of the bar at the starting position, and you think about this only at the start. You move under the bar as soon as it passes the top of your head, and this is what you think about from that point on.
The forearms are vertical when you think about putting your elbows just in front of the bar, and they only "slant" if this forward position is excessive. But you have to think forward or you will end up behind the bar.