The person you replied to was talking about pseudoephedrine, which is marketed as a decongestant. And boy does it get the job done.
Benedryl (diphenhydramine), on the other hand, is an anti-histimine and it absolutely works for allergic reactions. That said, it’s also marketed as a sleep aid (take a look at the active chemical in things like “ZZZQuil” or drug store brand “sleep aid,” and compare it to the one in Benedryl. Same chemical), and believe me, it works very well for that.