The e in eGFR stands for “estimated”. So it is an estimate of one aspect of kidney function - the glomerular filtration rate. It’s a very important measure and is usually reliable, but the FDA or a journal might require the actual GFR to be measured as proof of the effect. This is technically more complicated, as it involves an intravenous infusion, blood samples, and urine collection. 

Perhaps the silence from RVX on eGFR is because they are doing this is in a subsample of patients as confirmation.

It would make sense but it’s only speculation on my part.