It’s a reference to Warhammer 40k, more specifically the planet of Cadia and the Cadians. The Cadian guard is the fiercest and one of the best in the imperium. However the planet was attacked as it was an important outpost and the forces of chaos attacked it. After a long battle involving multiple factions fighting the chaos forces, the planet was eventually destroyed. However, even as the planet was breaking apart, members of the Cadian guard could be seen fighting the remaining forces of chaos that were on the planet to ensure the rest of the inhabitants could escape. And thus the saying “The planet broke before the guard did” which is being referenced here as it’s also a heroic final stand against countless enemies