If you read this carefully, you discover that he has only a single experiment in California to support his claim. The others he mentions are experiments where temperature was raised but CO2 was not. His one experiment was specifically on "the kinds of plants that live in California grasslands" but he is pretending that the result generalizes to crop plants in general.
There have been many experiments showing increased crop yield for increases in CO2 well above the current level. Examples include:http://scienceandpublicpolicy.org/images/stories/papers/originals/water_stress.pdfhttp://www.fao.org/docrep/w5183e/w5183e06.htmhttp://www.plantsneedco2.org/html/PositiveEffectsOfCO2OnAgriculture2.pdfhttp://www-personal.ksu.edu/~vara/allen-lh-ecsc04.pdfhttp://journal.frontiersin.org/article/10.3389/fenvs.2015.00048/pdf
A point he does not mention is that the effect of CO2 on yield is different for different plants. It is relatively large for C3 plants, considerably smaller for C4, the difference being in the mechanisms for photosynthesis. Maize is a C4 plant--the most important crop for which CO2 fertilization is a relatively small effect (sugar cane being another). Rice, wheat and soy beans, the other three of the four most important food crops, are all C3.
Nuccitelli tries to make it sound as though only global warming skeptics believe in future effects of CO2 fertilization. If you read the literature, you will find that scholars estimating the effect of future climate change take it for granted. To take one example at random:
"For wheat, we project small or occasionally negative effects of mitigation for projected yields, because of stronger CO2 fertilization effects than in maize,"
(Estimated impacts of emission reductions on wheat and maize crops, Claudia Tebaldi and David Lobell. http://link.springer.com/article/10.1007/s10584-015-1537-5
You might want to check that my comments are correct and adjust your willingness to believe the Guardian and Nuccitelli accordingly.