Having just read through it again, I also notice that the MQTT enable/install guide assumes an existing Mosquitto server (or similar?) is installed, installing Mosquitto itself doesn’t seem to be documented, except for in the emonSD build guide within the emonPi repo perhaps.
However that guide is specific to “Jessie” not “Stretch” and it not only specifies the 3yr old mosquitto-alpha extension, but also, in contrast, to install the cutting edge mosquitto package direct from the Mosquitto repos rather than using the Debian repo packages.