I've always seen scientism as a position (or stance) on the huge importance of science to philosophy and - often more broadly - to just about everything else. Thus I never took it to be a philosophy which advances independent arguments or positions on philosophical subjects. Indeed if it did do such things, it would be like a position within philosophy (such as anti-realism or physicalism).
Naturalism, on the other hand, is indeed a position within philosophy.
A distinction has to be made between what philosophers actually write (or argue) and their general attitude towards science. What they write (or argue) can't itself be science; though they can have an attitude towards science which people may call 'scientistic'. This was true of the logical positivists – and that's a point which they admitted to.
“scientistic people tend to be less concerned about actually consulting scientifically sound theories and more about being arrogantly in opposition to whatever they deem 'unscientific' -- notably religion”.