I doubt whether Marshall was referring to soldiers in logitiscal roles when he made his claim about only 20-25% of soldiers firing their weapons, but I do wonder whether other people are getting confused by those numbers. About twenty years ago I looked up what the "tooth-to-tail" ratio was for various branches of the U.S. armed forces, and found anywhere from a 1:10 ratio for the army (10 soldiers in support roles not expected to see combat, v.s. 1 soldier on the front lines who would be expected to need to fire his weapon), to a 1:25 ratio for the air force (which had, naturally, a lot more support personnel, such as mechanics and so on, who would spend their whole military career in hangars or on bases and never actually flying a single plane). That's anywhere from 10% to just 4% of military personnel, depending on branch, who would be expected to fire at the enemy; the only time support personnel would be engaged in combat is if something had gone badly wrong militarily and their supply lines were being attacked.

So while the article you linked isn't confused on the subject, and I doubt Marshall was mixing support personnel in with front-line soldiers in his numbers, I do wonder whether there are people who confuse those two numbers: the number of soldiers, sailors, coasties, airmen, or marines who would never be in combat even during times of war, vs. the number who would actually be in combat and not fire.

(The article did address "what if the battle never came near where those particular soldiers were standing?", which was the other question I wondered about).

On Killing further develops the idea [0] by looking at a wider set of battles across time and, crucially, finds that by adapting training methods, the kill rate went up to beyond 90%. This then appears to come with higher rates PTSD.

0 - https://en.wikipedia.org/wiki/On_Killing