How do you think spec sheets work? Engineers rely on data a d there are industry standards. That is the whole point of documentation. Even little motors and resistors have documentation that is relied on. You really think this is not documented accurately?
You really think that Optus is intensely checking and verify every device they sell? They rely on the documentation! They are a retailer of phones.
The way that Aussies think is always interesting. I find a lot of people bend over backwards to justify the reasons for companies. Instead of standing up for customers these arguments seem to look like a shining example of "out of scope" decisions. I have seen in too many corporate meetings and decision makers.
That is simply not true. What you get is standards. Standards bodies exist. In this most recent debacle all that is needed is compatible bands and VoLTE to make it compatible.
If these two exist, then it works.
Don't listen to me, here is a reference. https://www.whistleout.com.au/MobilePhones/Guides/Will-my-phone-work-in-Australia-carrier-network-frequencies