I don't think its so far off to say that PJ would use a Lewis reference. I mean, why not? Its a well known fact that Tolkien and Lewis were friends, and shared their writings with each other and the other Inklings. Personally, I think Gandalf was somehow reassuring Aragorn in a way. Making him say that Sam was with Frodo (therby covering Frodo's hiney
) may have made Aragorn, at least for a while, relax and trust that everything just might work out, even if the world was on the brink of meltdown. That's my take on it, anyway...