I'm struggling to find any kind of source to back me up, but I have seen this kind of thing before. It's equivalent to a breath mark, indicating a short break to let the sound clear and the phrase reset.
Why use the up bow/strum instead of the typical "," breath mark? I can't say for certain, but I can offer some guesses:
- Some people might view the breath mark as a literal indication to breathe, which of course does nothing on guitar.
- The breath mark can sometimes imply a slight break in tempo, whereas a bowing/strumming indication never does.
- Bowing/strumming indications are commonly used to force particular phrasing. In this case, even though strum direction is pretty much irrelevant, it could still be interpreted as an instruction to raise the hand in preparation of the next phrase, which implies a small break.
I consider it an abuse of notation, but a pretty minor one.