Possible in-universe explanation: I would guess people suspect that barring emergencies double-wizards keep their abilities secret. So Susan can use her double-wizard abilities just that once when she really needs to. A double-wizard would get in trouble for using their abilities all the time just as a wizard would get in trouble if they used magic around Muggles all the time. Also, what Harry does is so weird that it doesn’t even fit what they think a double-wizard might do.
Probable actual explanation: Eliezer didn’t think of the double-wizard initially.
The obvious conclusion is that Harry is a triple wizard pretending to be only a double wizard pretending to be a wizard (who sometimes pretends to be a muggle). Twisted indeed are the minds and deeds of triple wizards.
Possible in-universe explanation: I would guess people suspect that barring emergencies double-wizards keep their abilities secret. So Susan can use her double-wizard abilities just that once when she really needs to. A double-wizard would get in trouble for using their abilities all the time just as a wizard would get in trouble if they used magic around Muggles all the time. Also, what Harry does is so weird that it doesn’t even fit what they think a double-wizard might do.
Probable actual explanation: Eliezer didn’t think of the double-wizard initially.
The obvious conclusion is that Harry is a triple wizard pretending to be only a double wizard pretending to be a wizard (who sometimes pretends to be a muggle). Twisted indeed are the minds and deeds of triple wizards.