Unlimited data has FUP (Fair usage policy). It means they usually don't throttle you at all, but they reserve the right to if they see you abusing things. But generally, unlimited is unlimited.
Birdie on the other hand is fixed 12GB for $140 as you point out. After 12GB, you are 100% sure to be throttled immediately and its not a lot of fun, barely usable and really need to pay more to top up.
Don't get me wrong, I really like Birdie for mobile. I have two old $38 plans (cannot get anymore) with 300MB data for the kids, then I have another old $140 plan with 15GB data for the wife (also cannot get anymore, they reduce to 12GB). So total $216 which equates to $72 per person for 3 people and with the p2p function that Birdie has means 15.6GB data is pooled and can be shared among the 3 people. No admin charges, no contract, so I love Birdie for this. But as a home internet replacement maybe not the best because it doesnt have the unlimited FUP option.
HK Telecommunications companies are strictly business first, as almost all businesses are in HK. Unless the building is modern and a high rise it is unlikely to have fiber. There is no scalability for the telecommunications firm, hence no reason to offer it to customers. A prime example are residents in The Cyberport area. Those that live in multi million dollar town houses have complained about how their internet speed is lower than tenants in the high rise units right next door. There is simply no incentive in spite of the residents willing to pay the wiring, to provide high speed internet to the townhouse residents.