The crawlers' own disposable armor, temporary machines meant to be used up and then scrapped or destroyed.
Carl uses the Spider Tanks as his yardstick for the twenty Enemy Troop Carrier Tanks sitting idle behind the enemy line. If those tanks are real, he suspects they are the same as the crawlers' Spider Tanks: temporary pieces of crap that serve a purpose until they are either disassembled or blown to hell.
The judgment says as much about the crawlers' own armor as the enemy's — Spider Tanks are expendable tools rather than prized assets, a reasonable investment in a war where everything on the field is on a clock.
The crawler army's short-lived armored vehicles.