Considering Kuromorimine already fielded the frickin' Maus back during the TV series, it's just a matter of persuading the committee that the incomplete E-100 (the chassis was completed and waiting for the turret to be fitted when the Allied forces raided the production plant) was technically half-completed and still within the bounds of the Shenshado rules of allowing only WWII armored vehicles to be fielded.
Heck, the University team fielded the T95 GMC, which was even worse than the E-100. The Karl Gerat was a loophole abuse.
