Ann: Resource and Reserve Boost Adds Years to Lithium Project, page-22

Great spot - I noticed this also... I'd say the 12 year estimate in headline statements and then a casual mention of 15 years LoM is in keeping with JB's 'under-promise & over-deliver' mantra.

That said, the 15 year LoM was contextualised with mention of the stage 2 DFS. So perhaps, they have made the assumption that by the time stage 2 is in production that an extra 3 years (to the current 12) of ore reserve will be proved up... Again, they would only mention this prediction conservatively.
So as you've said, I would bet that actual LoM will be greater than this 15 year value.

Afterall, JB did mention systematically increasing the resource inventory... who knows, maybe could stretch out to a 20yr life by end of the project life!