Exactly what the article says.
If the vehicle has less than 8,000 miles, replace both banks.
If 8,001 miles, just the bank with a failed lifter.
Lowest mileage failure I've seen was 1,500.
They have a V.I.N. breakpoint in the bulletin, so that tells me they got a run of defective parts from a supplier.
My assumption as to the bank or all mileage thing is that they don't know if all the lifters are faulty and if only 1 side fails, see if the other does in time. They still have a ton of power train warranty left.
It's like their injectors. They have high, nominal and low flow. Company builds the rails with matching injectors. Gm pops them in engines. As long as all are the same, it's all happy. But when one needs to be replaced, we have to remove it to get the # off it to determine the flow. Put in the wrong one and DTC's will set.