I think running SLiRP on the Linux box and a SLIP client on the IBM AT was probably a stretch, but it's certainly possible. At that point it probably would have made more sense to grab an NE2000 NIC and throw the IBM AT onto the coax network.
I agree it's highly unlikely that the AT was running slirp. Wikipedia says an AT was a 286, so it wouldn't have been linux. Not even sure what the options would have been. Minix? Xenix?
I can confirm that they did run Minix OK, although I remember the network support was iffy at best. We never got it to work at any rate. XENIX would have been hard to get your hands on. I think QNX would run on an AT as well, although my memory might be playing tricks on me there.