I have that exact handrail. Ours is installed on the pool deck.
We put our pool in during covid, it was all that was available.
It came with those exact mounting bolts. And yellow plastic anchors.
I hope that is not what they used to install it, but it probably is.
I took ours to a local welding shop, who cut the large flange off, and extended the 2 legs. We used conventional railing pocket anchors on 8" centers.
The plastic anchors that come with that railing will never be adequate to properly install the railing.
Ill look tomorrow, I think I still have the flange that they cut off laying around. Ill post a pic if I do.