got this in SCMP:

"Passengers with an Octopus app on an Android smartphone can already use their handset to scan the QR code taxi drivers provide and make a payment. But those using Apple’s iPhone cannot because the device does not support phone-based transaction technology known as near-field communication.

However, from April these passengers will be able to tap their Octopus cards on a taxi driver’s phone, which will serve as a card reader through the tailor-made app. Passengers will also have an option to pay extra as a tip."

So it says otherwise.
What it means is actually we can now use our mobile phone to install a octopus reader and collect payment from another person's octopus card.. interesting..

technically if you can replicate that device in your mobile phone, then u can just walk around with the app on and hopefully pull some spare changes from octopus cards in the vicinity

and in the same article:

“Octopus has an edge. It is so widely used in Hong Kong,” Ng said. “But whether drivers or passengers will choose to use Octopus or others depends on what incentives it offers.”

which is what i have pointed out a page ago. in HK and Singapore for that matter, the battle for cashless payment is really tied to how much money the backer is willing to put in to incentivise users.. once the incentive is out, the user will just switch to the next guy that is offering something good.
i.e. there is no stickiness (come prove me wrong, tell me that you have stick to using a payment method after the initial offer and refused to changed to the next one offering new incentives) to any method.

and hk market itself is too small for these guys to really make money.