I wouldn't call 12.5" standard length for the Magpul. That the length without any spacers. Other than telescopic AR tube stocks, the only other widely available 12.5" LOP stock is the youth Hogue and Mesa Urbino. 12.5" is hardly a standard LOP for shotguns and the market is far greater for 13.5" and 14.5".
12.5" is designed for youth and with the use of tactical chest gear and plates. Which guess what...turns that 12.5" length back into a 13.5" or 14.5" equivalent once that recoil pad is pressed against that 1-2" thick chest gear..
Forend length and positioning on the gun also plays a big role.
Shotgun fit is personal and definitively saying 13.5" "sucks " for " "modern" stance is extremely short sighted. It may suck for you. But you are not everybody.
This is a great video on LOP with the SGA stock. I love the SGA I have four of them on four different guns, but none of them are without spacers. Just personal preference.