From Merriam-Webster's definition of foodstuff is the following example sentence:
// Hyperinflation has put all but the most basic foodstuffs and medicines out of reach, leaving those who remain thin, hungry and sick.
— Washington Post, "In Venezuela’s oil capital, life is a struggle. So is death.," 20 Sep. 2019
The pluralization of stuffs isn't that much of a leap from the pluralization of foodstuffs.
There are also other manipulations of English that have allowed for this kind of pluralization.
One has an additional twist, because it pluralizes a noun that shouldn't really be a noun in the first place:
I have all the feels.
Another is a recent slogan from Arby's [YouTube video]:
We have the meats.
Mass nouns are seldom pluralized. However, there are exceptions.
From "What is the Plural of 'Money?'" by Merriam-Webster:
Like common mass nouns such as water and sand, there are occasions when money is inflected with a standard plural, like any count noun. It tends to occur when the reference is to discrete sums of money, obtained from a particular source or allocated to a particular cause.
The spelling in these instance can be moneys or, more commonly, monies:
Some of the 2009 stimulus monies for high-speed rail should end up in the California project and more may be coming.
—James McCommons, Waiting on a Train, 2009
So, the only real reason why stuffs could be considered incorrect is simply because it's not normally used. Since it isn't used, it doesn't sound normal. But that doesn't mean that it can't be used.
Repetition has made all the feels and we have the meats sound somewhat normal. Or at least more normal than when they were heard the first time …