True, but there are variables:
A little ash fall will be great fertilizer.....say, a dusting.
A little more and they will inhibit growth and size of harvest.
A little more (more than an inch) will kill plants, and the harvest, but can be skidded/scraped into a new planting for next season.
A few inches will not only kill the crop, but be very problematic in disturbing the dirt and soil composition, specifically chemistry.
Drop a foot of ash? I cannot even contemplate the vast amount of resources that would be needed to rehabilitate the area for quick resumption of farming.
The ash is not that great a problem. The amount? Yes. I see the climate as a greater problem.
See..the year without a summer, 1816, etc.
That eruption, disrupted climate over the whole world.
In effect, those who die directly as a result of a mega eruption might be the lucky ones. And Tambora, was TINY compared to historical Yellowstone eruptions.