There are always a lot of special conditions and waivers for policy changes like this. He could have had 18 years in already, and the policy allowed airmen like that to still retire.
The Army was not a popular career option in the early 1970s, not least because of the possibility of being sent to Vietnam, plus stories about drug use and racial antagonisms which circulated in the Press. Recruiters really struggled to find quality EM candidates who positively wanted to join. I used to know a fellow living in England (he’s gone back home now) who joined the US Army around that timeframe and wanted to, and the recruiter grabbed him with both hands, so to speak.
His specialty was tank mechanic and by that time the involvement in Vietnam was winding down to a close and they were pulling people out, not sending in more. Plus they had very limited use for armor in that theater. So while in theory he could have been sent there it wasn’t very likely.
While there have been some examples of people disabled in the service who continued to serve - Admiral Nelson, Air Commodore Freddie West, VC, Douglas Bader (rejoined, in his case) they are more the exception than the rule.
During the mid 80’s my company Sergeant Major, who had lost part of his leg in Vietnam and was a lower leg amputee. I heard he had broken his prosthesis during a jump and had hopped off the DZ. I wasn’t there so I don’t know for sure, but I would bet it happened.