IRB::OutputMethod#pp is actually more like Kernel#p than PrettyPrint#pp. There doesn't seem to be a good reason for the misleading name, and use of OutputMethod requires all the Kernel print methods. (Predicated on #9876 being fixed, of course)
This definitely isn't a bug, and I don't think it's worth breaking backwards compatibility for this consistency, so I'm going to reject this. If you would still like the change made, please submit a pull request to https://github.com/ruby/irb and @aycabta (aycabta .) can determine if he thinks it is worth changing.