From an experience perspective onboard it is premium economy.
For European flights the cabin started it's life as Economy Flex, Long haul was Economy Extra, though both became Economy Extra and then eventually Plus.
The lounge and fast track aspects are clearly business class elements, but where already there when European short haul where still 3 class.

In some ways, the start of Plus reminds me a bit of the start of Euroclass/First Business Class when SK targeted themselves as the business man's airline in the late 70ies early 80ies, they took some of the elements from first class with some upgrades of the economy product and packaged them in to the expensive flexible economy fares and invented a new cabin class with the movable curtain. Voila, Euro style business class was born from a concept of putting a class between economy and the European first class cabins.

From certain practical booking perspectives, and from a cabin of service perspectives to get partners to recognise the lounge access, it is flagged as business class. Considering the flexibility in systems, and unbundling of fares it would probably be easier to manage the lounge access without the business class moniker, but if is not broken, why try to fix it?

It is a hybrid product, and it does not really fit either bucket. But I think seeing it as PY sets the expectations more accurately.