The description of the getBindGroupLayout method of GPUPipelineBase doesn't really say what happens when index >= this.[[layout]].[[bindGroupLayouts]].length.

The [[bindGroupLayouts]] internal slot has type list<GPUBindGroupLayout>. The list type constructor is defined in the Infra Standard, which simply says,

The index cannot be out-of-bounds, except when used with exists.

so the meaning of the expression in the getBindGroupLayout algorithm when index is out of bounds doesn't seem to be defined.