As you know, infinite period grating creates diffraction at isolated diffraction angles. So your result is correct.
As for the published result, please check the paper again. I suspect that its incidence is a cone beam, such as from a microscope objective. because of broad incident angles, it may induce continuous angular distribution. From your grating analysis, the 1st diffraction has diffraciton angle of 30deg. therefore, I guess the incident beam would have about 30 deg cone. Please check.
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