That's how models and rule work in general. There's a rule or pattern that mostly applies but always exceptions.

In this case, the old systems were all torn apart by older hackers. There's young ones, just a few, hacking mainframes and stuff now. The ease of finding first flaws showed only reason they weren't found sooner is nobody cared enough to look. Or couldn't afford the relics to play with. So, believing one is safe just because computers are old is akin to Security via Senority.

Now, there are older approaches like tagged HW or dedicated lines whose intrinsic properties reduce odds of attacks. Definitely apply any good techniques from past to present situation. Just that it's old... especially since it's older than INFOSEC itself... doesnt make it safer.