I think the term is being used slightly out of its original context which is one of beancounting. I came across a similar usage in relation to the stock rundown of single-use plastic items:
"Le décret fixe en outre un délai d’écoulement des stocks de 15 mois pour les produits mis sur le marché avant l’entrée en vigueur de l’obligation, pour éviter la destruction de trop nombreux stocks de produits ne disposant pas du marquage, et pour lesquels l’apposition d’un autocollant ne serait pas possible."

In this case, and yours I expect, this is the time items can be held in inventory before holding them is prohibited. In the normal sense, it is the time they are held before they are sold (the stock being replenished all the while).

"Inventory Days also known as Days Sales of Inventory, is a financial ratio that shows the average time in days that a company takes to sell its inventory."
https://www.linnworks.com/inventory-management-software-glos...

"The days sales of inventory (DSI) is an important financial ratio and metric that helps indicate how much time in days that it takes a company to turn its inventory. The ratio also includes any goods that are still a work in progress. Essentially, it measures how efficiently a company can turn the average inventory it has into sales.
It can commonly get referred to in a few other terms. These include the average age of inventory, days sales in inventory, days inventory, days in inventory (DII), and days inventory outstanding (DIO)."
https://www.freshbooks.com/glossary/financial/days-sales-of-...


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Note added at 1 day 20 hrs (2023-02-05 10:35:52 GMT) Post-grading
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@ Francis Boye (see Peer comment): It's not me who says it, but I agree it is not a 'financial ratio' but rather a number that is used in accounting (finance).
I suppose it could be called a ratio if you consider it is a number of days in relation to a number of items; if 100 items are sold in 20 days you have a ratio of 5:1, but 5, derived from that ratio, is the average number of inventory days. You might have sold 50 items on day 1 and 50 items on day 20, but the average (of the ratio over that period) is 5.