How did you come up with that? nothing in the code says draw an imaginary line, it does say the minimum threshold is 2" above the finish drain.That would not be how I interpret UPC 408.5. I think that with a curbless shower you get to draw an imaginary line that defines the outlines of the shower, consistent with the minimum size requirements. Often it's pretty clear where to draw that line, but with a "shower room" type install as in the OP, it's not at all clear.
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Chapter 4 Plumbing Fixtures and Fixture Fittings: Plumbing Fixtures and Fixture Fittings, California Plumbing Code 2019 | UpCodes
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Note that 2018 UPC 408.5's last sentence says "the immediate adjoining space to showers without thresholds shall be considered a wet location and shall comply with the requirements of the building, residential, and electrical codes." So that raises the question of how far the wet area should extend, particularly in a shower room like in the OP. One could argue that the whole room should be considered a wet room.
Of course, we don't know if the OP is in a jurisdiction that has adopted the UPC rather than the IPC or one of the other possibilities.
Cheers, Wayne
"In no case, shall a dam or threshold be less than 2 inches (51 mm) or exceeding 9 inches (229 mm) in depth where measured from the top of the dam or threshold to the top of the drain".