Not surprising to see F₂⁺ & HF⁺ being observed; as a chemist it's what I would expect from a LiF system being bombarded with positrons. Really I'm surprised that it hasn't been tried before! Although that probably is on account of positron beams not being a tool available in any department I've known.
It's not that hard to make a positron source for that kind of experiment as you can just use some radioisotope that produces them by decay and you don't need a big accelerator.
Lots of scientists I've known tend to avoid radioisotopes, given either the inherent risks or the perceived risks. University bureaucracies can also create disincentives and barriers, even when the related procedures well known. The paper's paywalled for me, but I'd be interested to know what approach they took to generating the positrons and the beams.
I think it's pretty crazy that they found chemistry that can be driven by positrons (as opposed to just subatomic reactions)