Howdy,
I'm not sure what you mean by an offline installer, but the available updates, the admin guide, and the release notes are all correct. 10.1.5 is an update (MSP). It is not a full installer. As stated in the doc, full installers are not always provided. Also, full installers or updates don't have any online dependency.
You can install 10.1.5 over that last relevant msi/exe. That's 10.1.4 for some languages but usually 10.1.
Alternatively, you can get a non-enterprise installer from the Reader Download Center. You can probably expand the exe to get to an MSI. That's an atypical route most enterprise admins have now found a need to travel.
10.1.6 will probably not offer a full installer either, but we'll see.
hth,
Ben