Sure, you could install iPhone and Android smartphone versions of Duo on your tablet already, but the formatting was awkward on a larger device. Duo now takes better advantage of the extra tablet space, particularly if you're doing a group video message. The iPad version is definitely new, but some Android tablet users have had it now for a while, so Google apparently did a limited rollout earlier this summer.
Duo has become the default Android calling app, while Hangouts is now relegated to business use. Duo started off as a rather bare-bones app designed to work with poor connections, but Google has gradually added features like screen-sharing and, most recently, multiple device support. Companion app Allo hasn't garnered the same attention, but Google seems to have big plans for it.