It's a quarter of the graph (hence quadrant)

The choice of X-axis maximum and Y-axis maximum dictates the cutoff point of the quadrant. With a maximum of $80 per task, the quadrant is $3.1 and lower. If some new model gets released and costs more than $80 per task, the "most attractive" cutoff moves up from $3.1 since they have to move the X-axis max. It's completely arbitrary.

It's not saying that being inside that quadrant is a hard cut-off it's just illustrating the direction of the pareto frontier

It is suggesting a hard cutoff since it says "most" attractive, a superlative, instead of "more" attractive. And it is not pareto optimal, because if the models get really good and are scoring 10x more for 10x cheaper that green region will no longer be pareto optimal.

Of the 4 quadrants it is the most attractive

I suppose you’re not wrong lol