Most of the artillery Ukraine is firing uses soviet calibres. 152s and 145s. The article conflates the potential scarcity of that ammunition with the lowish production of NATO calibre shells in the US, but given how little NATO-standard artillery Ukraine is using, those US-manufactured shell stockpiles won't be going Ukraine. If they go anywhere, it will be to NATO members who have sent their own soviet-era equipment to Ukraine in exchange for newer NATO standardised equipment. The article is also not differentiating between which systems are showing possible shortages. It's actually pretty useless for information purposes, but that's not a surprise, because journalists are retards who rarely understand their subject matter.
This entire story is based on an analysis by someone called Mark Cancian, who is formerly a lot of things but currently works for a think tank that is closely linked with the "industrial" part of the MIC, and has a vested interest in promoting an increase in government spending on weapons. The link to his analysis is dead, so there's no way to verify his numbers.
Despite your characterisation, this isn't a warning that NATO is stripped bare. This is the MIC lobbying, through connections with journalists, for the government to start buying more of their bombs just in case, backed by some likely made-up numbers and a bunch of wild over-estimations about Russia's manufacturing capacity.