explanation is pretty easy
the US needed a "win" after the absolute clusterfuck that was afghanistan retreat, so they pull out of some dusty drawer another "security arrangement". these are the easiest things to sell, appear important, seem like it's a progress and tightening of relations, when in fact it does diddly that is in any way new or meaningful.
for boris johnson, he needed to convince the uppity brits that hey look, we are still a pacific power amirite guys oi oi, rule britannia. giving the finger to the french was a bonus, as it always is for the brits. they probably were the ones that insisted on the secrecy of the matter.
for the aussies, they are just happy to wag their tail towards the bigger brothers of the anglosphere, america and GB.