The extra 100 manpower they get is to compensate the manpower cost they added to trucks, can't be more clear and precise.
Let you check post 102
100 manpower added at the same time SWS Trucks became constructible. from 240 to 340. 240 was enough to build your first volks squad or kubel right off the bat.
That sounds simple to fix: cut 100 MP off all the truck setups, remove the extra 100 starting MP.
That weakens OKW's opening, but saves them 100 MP on the SPHQ and makes their sideteching/truck rebuild cost 200 MP instead of 300 MP.
You'd need to adjust Bolster and the M3 Scout Car at the same time. Possibly shift the M3's cost from fuel to manpower and rework Bolster into a 60 MU squad upgrade.