You're making the leap from "booking.com is a cause for the bug" to "booking.com is the only cause for the bug".
The booking.com app is/was absolutely one of the triggers.
I am not making this leap. He said he can confirm this issue is caused by the booking.com app. It's not. It might be a trigger, or it might be one of the causes. But booking.com is not THE CAUSE of this issue like he said.