"explicitly on a stack" in the sense that Forth directly exposes the presence of the parameter stack to a programmer, in contrast to the stack that, say, a C program uses.

"passed implicitly" in the sense that Forth definitions do not require a programmer to explicitly state which arguments are passed in and which results are yielded. (although many Forth programmers provide "stack pictures" in comments by convention.)

OK, I see. I consider myself corrected.

No, it's definitely implicitly.