So, the DoD distinguishes between service awards and Personal Military Decorations (PMDs). Service awards include campaign medals, good conduct and longevity medals/ribbons, etc. These are generally awarded on a non-discretionary basis, i.e. if you meet the criteria for the award, it's yours. You can even go back years after you left the service and if you met the criteria for an award that wasn't put in your records they will correct it for you.

PMDs require a nomination from your chain of command, and can be given for any number of acts deemed worthy of recognition. The act being recognized will be unique to the individual. The lowest PMD is the Achievement Medal, followed by the Commendation Medal. Each service has their own version of these medals, and there is a joint service version as well. Although these awards aren't given automatically, it's the custom of the service for members to get one every few years, with the grade of the medal generally correlating to their rank. For a junior enlisted member who only served one enlistment, you would expect to see either an Achievement or Commendation Medal. The Marines are known for being stingy with medals, so it's not an immediate red flag that all he got was a single Achievement Medal during his service.

It's customary in official biographies to only list PMDs and omit all the awards below Achievement Medal, so Wikipedia MAY be following that standard. Nonetheless, the timing for the edit is highly suspect and I would want to see if the same edit is being made to other military figures that have their decorations listed on their Wikipedia page. I certainly wouldn't default to giving them the benefit of the doubt.