I suspect you're right. Currently, DirecTV satellite offers a traditional set-up with the HR-44 and HR-54 Genies, as well as the newer server + client set-up with the HS17 server and C61 Genie Mini. My guess is that the next generation of hardware will be server + client only, i.e. HS27 and C71. Regardless of the branding/marketing specifics involved (including channel packages, pricing, etc.), it seems very plausible to me that the C71 will be available to subscribers who get AT&T video service by either satellite or OTT streaming. In the former case, the C71 would have to be paired with the HS27 server, which contains the satellite tuners and DVR hard drive. In the latter case, each C71 could stand alone, connecting via internet to the AT&T video cloud serving up all live, recorded and on-demand TV. There appear to be different versions of the C71, which may correspond to whether that model is intended for use with a satellite or OTT source.
That's speculation, of course. Maybe the C71 will only be for satellite customers, despite the fact that there's been lots of talk, including from AT&T's own CEO, about having a dedicated thin-client STB for use with OTT streaming. OTOH, maybe the C71 will only be for OTT streaming customers, although if that's the case, it's a little odd that the manual submitted to the FCC for the C71 mentions that it can work in conjunction with the future HS27. And clearly, C71 and HS27 are model numbers consistent with the next generation of DirecTV satellite hardware for the Genie Mini and home server.