There isn't much published information about these systems to my knowledge. This[0] report from ANS-Net[1], though, indicates to me that AIX was very much directly involved in the forwarding part of the router architecture:

  AIX Kernel Radix Trie problem

  3+ overlapping routes with different masks

    abc.def/8
    abc.def.0/10
    abc.def.0/12

  Fixed in AIX build coming within two weeks

While it would certainly be possible (and is a common router architecture now) to have a central forwarding database and distributed line cards, it doesn't really make any sense to use a kernel trie to store routes for such an architecture, and if the central rs/6000 was switching every packet, it wouldn't - at the time - have been able to support a single t3, let alone the kind of port density described in the original article.

So, I guess in summary, I can't point you to definitive proof that those boards were running AIX, but it seems extremely likely to me.

0 - https://www.ietf.org/proceedings/29/ops/netstat.becker.slide... 1 - A network operator that was subsequently purchased by AOL.