Yes, return can be a function. Torvalds hasn't heard of continuations, obviously, where we "return" from a function by invoking a continuation.
We can justify writing return (expr); using the same arguments that justify the sizeof (expr) convention.
The thing is that in C, return isn't a function; there are no continuations.
Similarly, sizeof is an operator, which doesn't reduce its argument expression to a value.
If we are going to make coding conventions based on pretending that C is a different language in which sizeof is a function, then pretending return is a function is also fair game.
That point is clearly made in the comment you're replying to, with the additional point that if we are talking about C, then sizeof is an operator, not a function. It doesn't require parentheses, except when its operand is a type expression. No well-considered coding convention requires superfluous parentheses with sizeof, and there are good reasons to ban them.
We should prefer code like:
type *ptr = malloc(sizeof *ptr); /* no parens */
to
type *ptr = malloc(sizeof (type));
In general it's often better to base sizeof on an ordinary expression rather than a type expression.
If we don't use superfluous parentheses on sizeof, we can then look for sizeof followed by an open parenthesis to look for code where sizeof is applied to a type expression.
sizeof (type)
means "produce me a size_t value based on some arbitary type, without checking that it's related to anything in the surrounding code". It can be as dangerous as a (type) cast.
Scheme has come up with the idea of pretending that an undelimited contination is a function, which is somewhat true-ish... in the degenerate sense of a function with empty domain.
This is fine for Scheme, which is an expression language, so every method of invoking a continuation would, syntactically, be an expression anyway.
In C, which has statements, making return look like an expression is somewhat pointless. It doesn't matter whether C "has" continuations (and it arguably has) or not.
We can justify writing return (expr); using the same arguments that justify the sizeof (expr) convention.
The thing is that in C, return isn't a function; there are no continuations.
Similarly, sizeof is an operator, which doesn't reduce its argument expression to a value.
If we are going to make coding conventions based on pretending that C is a different language in which sizeof is a function, then pretending return is a function is also fair game.