Citation: refer to author or paper?

Supporting @user153812, I'd write

Marx and Engels (1888) propose sharing property ...

but

The paper (Marx and Engels, 1888) provides the first definition of communism ...

The rule of the thumb is that a citation in parenthesis should not disturb the flow of the sentence. Every time this not happens, I find myself referring to the authors, hence the plural. The only exception is when there is a sole author.

Lenin (1907) provides a crucial extension of the initial suggestion of Marx ...

I had to correct myself, having written it initially in past tense, which sits better with my belly felling, but does not provide a good reference to the genus.

(Note that I did not proof-check the dates, it's just the May spirit.)

In literature I generally see references to people much more than to source. When faced with ambiguity, I prefer choosing what sounds more correct grammatically, even if the difference is slight.

In the example above, it should be 'X and Y (2011) describe', because the verb 'describe' would apply to an animate subject (person) rather than an inanimate subject (source). Similarly, demonstrate, suggest, propose, hypothesise would be apt for people.

The source would be better referred to when using 'inanimate verbs' like contains, comprises, constitutes etc.

Examples:

X and Y (2011) propose that trees be considered sapient.

X and Y (2011) contains compelling evidence for trees to be considered sapient.