Negligence requires a breach of duty. In this instance, the gym has a duty to provide a safe and well-maintained facility. If the damage to a piece of equipment is superficial and non-structural, and the cost of replacement is excessive, then it wouldnâ€™t be considered negligent to not replace the equipment (the risk is low and the expense is high). In this instance, the bar is only chipped on the laminate, the internal fiberglass which amounts for the actual load-bearing capacity of the bar isnâ€™t damaged. Therefore, the structural integrity of the bar remains intact. Filling in the laminate is more akin to giving it a paint job than anything else.
Furthermore, industry standard and common practice in the community would reveal that equipment is often repaired by experienced coaches. There isnâ€™t a single gym in the world that doesnâ€™t have some home-made equipment, whether parallettes or other floor bars, or a raised platform, etc. If youâ€™ve ever anchored a bar into the ground yourself (as opposed to by the manufacturer delivering it) then youâ€™ve â€œmodifiedâ€ the equipment yourself.
Bottom line, if you read the passage on the original blog, this repair was done with careful consideration for the safety of the athletes (taking into consideration a child-safe sealant, sanding for evenness along the rail, etc.) which would hold up as being a reasonably prudent and justified repair.
Factoring in all this together, the gym could prove they did not breach their duty because they took reasonable precautions and assumed an appropriately low risk.