He was definitely using those in the Ultra Herc later on! But they're only marginally better than Trebs for killing Sath beams anyway (They're higher subsystem DPS if you spam them by switching banks, lower DPS if you wait out the full reload time)
The gold standard of course is the Helios, but those weren't available - due to a bug, maybe? Imma rip this open and check
The "Incursion" campaign does not include the Helios in its initial secondary list, and Sublime Text tells me there is no allow-weapon SEXP for the Helios anywhere in the campaign. (There seem to be allow-weapon SEXPs for weapons that are already in the default list?

)
The "Return to Sol" campaign does include the Helios, and - funnily enough - so does the combined campaign that includes both in one. So if Joe had played using the "Incursion & Return to Sol" campaign file, he would have had Helios available from the start.

I'm don't think the combined campaign file is original - looks like it was added by Macfie for the FSCRP, so not having Helios available might have been the original intent. But it's a pretty dumb intent tbh
EDIT: also the Ultra Herc would not have been able to carry Helios