It's pretty clear they understand that. Creatinine is, however, the waste product of creatine. So the more creatine you consume, the more creatinine is in and discharged from your system.

Precisely and correctly as they said, normal eGFR presumes average musculature and average creatine consumption. If either of these out of the norm, eGFR becomes inaccurate and potentially flagging false positives for damage. Creatinine, the waste product of creatine, raises in a way that can get confused with kidney damage, which is precisely how the confusion about it causing kidney damage or being bad if you have a compromised kidney came about.

In some studies, people with CKD actually improved with creatine supplementation, though notably this was not people with PKD where it could increase cyst growth.