Either is fine because product can serve as a count noun or mass noun, and the subject complement construction allows for either form.
The structure we're dealing with is:
A are not the only B ...
A is a plural noun. B is a noun that denotes what kind of thing A is. This is a recurring structure in English, with 842 results in the Corpus of Contemporary American English (COCA) to work with. So my answer to this question may also explain some portion of the sample that use singular B with plural A. (A quick count suggests that out of the first 100 entries, 38 have singular B and 62 have plural B, so B is singular at least sometimes.)
Why, then, would product (or a similar noun) be valid in singular or plural form?
In this structure, B is a subject complement to A as connected by a linking verb, modified by an adjective (only, which customarily takes an article the, a possessive pronoun, or a possessive noun in this usage according to the Oxford English Dictionary), which is in turn modified by a negating adverb (not).
In simple examples of the subject complement found in grammar guides, when it is a noun, the number of the subject complement corresponds to the number of the subject:
Brandon is a gifted athlete. (Singular -> Singular)
He became a famous writer. (Singular -> Singular)
Both the brothers became doctors. (Plural -> Plural)
That's often true when we're dealing with count nouns, or nouns which describe a countable quantity of something. However, some nouns act as noncount nouns or mass nouns, which cannot be counted. A mass noun does not take a plural form, so when it serves as the subject complement of a plural subject, it remains unchanged:
Bananas and apples are fruit
Puppies are happiness. (Seen in the wild in this product description.)
Some mass nouns are always mass nouns. Other mass nouns are sometimes mass nouns and sometimes count nouns, or they can be converted into a count noun through usage (will you order me coffee? -> I'll have two coffees). If you read fruit and thought that making it plural would be acceptable as well, you're right, because fruit also sometimes works as a count noun to describe pieces of fruit or kinds of fruit:
Bananas and apples are fruits.
Mass nouns also sometimes convert to count nouns. In the article "The Lexical Semantics of English Count and Mass Nouns" (1999) author Brendan S. Gillon states four denotations (meanings) when mass nouns are converted to count noun:
- To be a kind of
- To be an instance of
- to be a unit of
- To be a source of
In mass form, this is often their purpose anyway, but as count nouns they can now indicate a countable kind of/instance of/unit of/source of something. So when I say "Puppies are happiness," I'm saying puppies are an instance or source of happiness. When I say "Bananas and apples are fruit," I'm saying that bananas and apples are kinds of fruit. When fruit is converted to a count noun, "Bananas and apples are fruits" suggests that they are kinds of fruits and also that they are countable instances of this. The subject complement allows for this kind of flexibility between count and mass nouns.
So, back to product and products, either one is acceptable because product can operate as both a mass noun and a count noun in a similar way to fruit. As a mass noun, product describes the kind of thing capstans are: they are a kind of product, and "we" produce other kinds of product. As a count noun, products corresponds with capstans because (in mass noun form) capstans are a kind of product, and (now in count noun form) capstans are (countable) products. The end result is a lot of nuance between two example sentences understood in similar ways by fluent readers:
Capstans are not the only product we manufacture.
Capstans are not the only products we manufacture.