-Under Secundus Incursion Gangs the relevant points are that first you choose a base gang, in this case Escher.
You can take their fighters, equipment etc but the important part is "and it is treated as the base gang in addition to being a secundan incursion gang.
-Next it describes that instead of their normal leader, you use either an orrus, malcadon, jakara or yeld spyrer. "This fighter type changes their type to Leader and gains the Leader Apart and Secundan Incursion Gang Leader Rules.
Under the core rulebook on fighter types there is a section about disregarding the name of the fighter for its type. An Escher queen, van sar prime, orlock captain etc are all considered as Leaders and says this rule is universal to all such models regardless of their type