Demarchy has not an agreed upon meaning.
Friedrich Hayek first used the term demarchy to name his proposal of a (according to him) better form of government. He used the term because democracy, in his opinion, is now being used to name forms of government that have not much to do with what it originally was meant to describe. His proposal in no way included the use of sortition.
John Burnheim, an Australian philosophy professor, was the first to use the term in relation with sortition. In his book Is democracy possible?, he envisions a form of government that among other things includes the dissolution of the state, and indeed also a large number of citizen juries, selected by lot, that take decisions on public policy issues. He calls his new form of government demarchy. See here for more details.
I'm not sure exactly how, but very recently a number of people online have started to use demarchy as a general term used to mean "sortition based democracy". One of the first has been the wikipedia article on demarchy. Likely others got it from there. Most people however, specially those really studying the subject, do not use use it this way (usually they do not use it at all).
Sortition, in the context of politics, means just allotting any kind of position by random selection.
Given the ambiguity around the term demarchy it is, in my opinion, much better to talk about sortition.