Just because someone says something with such conviction, doesn't always make it true.
I'm not saying he's wrong, because he may not be. But there's more to it.
Underpitching does not always cause undesirable off flavors, so your beer may come out great 99% of the time; however, saying that it's ok to under pitch by 50% simply isn't true. Underpitching is done deliberately in an effort to produce off flavors that are desirable in particular beers. Not all beers benefit from such flavors. I under pitched my saison on friday. People will often under pitch for hefs too. They want to bring out the esters, banana, clove.. etc.
Diacetly can be produced for several reasons-underpitching, long lag times, stressed yeast... It can even happen due to infection.
We don't know if you're even tasting diacetyl. We're just assuming. It could be something else you're actually tasting, but to assume that diacetyl was produced due to a lack of sufficient viable yeast is not far fetched especially when working with a yeast that is known for diacetyl production.
He is correct in stating that it could be something else. Often times it requires a "perfect storm" for something to occur-meaning it's not simply one thing that caused an issue. Because of that, it can be difficult to pinpoint the exact problem. It can be especially difficult when trying to get someone to explain their exact process to pin point an issue.
You could've under pitched, you could've under oxygenated; your temp controlled environment could've been off in temp; you could've made your yeast starter months ago then refrigerated it which would lower the viability of your yeast; you could've made yeast starters for way too many generations basically mutating your yeast.