As the parent notes though, "the passive also allows you to choose whether you want to call out the source of an action", while "they merged the pull request" does not.
They mean it allows you to not point out who did the merge.
If the PR introduced a BUG or was a bad choice to merge it, you might not want to use an active form, because then you have to say who did it (for example, in a report), to protect them, or to not appear like you're pointing fingers. The passive voice allows you to state the fact that the commit was merged, without mentioning anybody.
"John/Jil merged the pull request"
vs
"The pull request was merged" - you can chose whether you want to include "by John/Jil" or not.
