How is this a backcross?

I have the Grape Stomper OG bx which is

Grape Stomper OG × Mac Stomper (Mac1 × Grape Stomper OG)

Wouldnt this be an F1?

The way the notation reads is like Female x Male, so GSOG_f x (MAC1_f x GSOG_m) is not a backcross according to that notation. To be a backcross, the GSOG would have to be from the same mother plant (genetically identical). If it were a backcross, then it is the BC1, BX1, or N1 generation, but the way the notation you posted reads is that the seed is an F1 generation seed.

For backcrossing, the backcross lineage should be genetically identical, but sometimes the term is loosely used with genetically similar individuals.

Sometimes different letters are used to represent the different generation relationships, but those examples are ones I've seen used before.

Good luck! Peace