Is a syscache tuple more like an on-disk tuple or a freshly made one?

Please bear with a neophyte question ...

I am tempted to apply HeapTupleGetDatum to a tuple retrieved from
the syscache (as I already have code for processing a tuple presented
as a Datum).

But I see a comment on HeapTupleHeaderGetDatum: "This must *not* get
applied to an on-disk tuple; the tuple should be freshly made by
heap_form_tuple or some wrapper ..."

... and here I confess I'm unsure whether a tuple retrieved from
the syscache is more like an on-disk one, or a freshly-made one,
for purposes of the warning in that comment.

Is there a conventional proper way to pass a tuple retrieved from
syscache to code that accepts a tuple as a Datum? Or is there some
fundamental reason a smart person wouldn't do that?

Thanks,
-Chap