Whether Neil Armstrong was a deist or a Catholic, he could not have participated in a Presbyterian communion in good conscience. If he were a member of the United Church of Christ, he could have, as both groups hold to a similar, Reformed position that the Lord’s Supper is symbolically, but not physically present in the bread and wine, rather than physically present in the elements (Lutheran) or actually transformed into Christ’s Body and Blood (Catholic).
Is that a deist belief though? As a Catholic, I know a Catholic is not allowed to participate in a Presbyterian communion service.
Having said that, it appears that there are conflicting reports on the internet regarding his religious beliefs. I have seen Christian (with no specific denomination), deist, as well as no religious affiliation. I was unable to locate any that state he was a Catholic.