this is on a plant running already for many years and still they can get so much more ore profitably recovered ?

No, they just built a new mill costing a great deal of money:

HVC parent Teck Resources Ltd. is investing more than $600 million at the site for three projects that began last year, including searching its property — including pits mined in the past — for more ore, bodies that could extend the mine’s life past its 2027 expected closure. About $540 million of that sum will be spent on mill optimization, the largest-ever capital investment made at the mine.

The largest project at the site is the new flotation building. Inside, twenty-one cells — a series of cascading tanks that separate minerals — will replace today’s 257 cells. Along with the pebble crusher and tailings improvements, the investment will make the mine a more efficient producer — all important when mining a low-grade deposit.

It is possible, or probable, that Teck is taking material from Schaft Creek and doing their metallurgical studies at HVC in their new mill. They've obviously spent a lot of work recently looking at improving grades in large porphyry mines and I would imagine they knew how far they could take the Schaft Creek grades.