Cellular chip antenna feedline width?
The width of the stripline is not a design target in itself.

You design a transmission line to have a specific impedance, to match the antenna or whatever is at the other end of the transmission line (or both, if possible).

So, you're right to calculate based on your actual substrate and copper properties. Both 1.3 and 0.3 mm are feasible widths, and variations on a scale of 4 really don't surprise me – there's a large difference in dielectric constants across different board substrates, and a large difference of board thicknesses.

So, make sure you read the datasheet correctly – I can't find your 1.34 mm anywhere in that datasheet, but a clear labeling that says "50 Ω transmission line".