Does "under-shipped" mean they are sitting on a warehouse full of inventory they are intentionally not selling, or does it mean they truly had a no or low penalty way of reducing production?
AMD, Intel, and Nvidia all make their products in pretty much the same way -- dies are manufactured in bulk in foundries, before being sent on to a packaging plant, and then finally moved to storage and distribution centers.
In the case of AMD and Nvidia, they need to place orders with TSMC months in advance (or at the very least, 4 or 5 weeks) and it takes around a month to go from a silicon ingot to a finished die. So rapidly slowing down or ramping up chip production isn't something that can be done very quickly (Intel can do this quicker, as they have their own foundries).
Judging exactly how many chips to produce is a complex matter and it's generally best to under-produce, rather than over-produce, to ensure that there isn't an excess of inventory just lying around. The market has shown over the years that it's willing to 'accept' a shortage of products, especially graphics cards -- every time AMD or Nvidia has released a new lineup, the popular models are always in short supply.
Contracts with retailers, OEM suppliers, and AIB vendors will contain estimated projections for how many trays of chips will be available in a given period and such figures are likely to be on the conservative side of things, as well as have caveats along the lines of 'market conditions will affect this rate of shipping.' So under-shipped in this context means that AMD, Intel, and Nvidia will have not met this rate.