Correct. DLR made the interesting decision the last time they replaced the monorails to not get A/C. I am not defending that decision, but the monorail is more of an attraction at DLR, whereas at WDW it is necessary transport to/from parks.
At DLR, if the monorail goes down, it is annoying and frustrating, but it mostly impacts guests staying at DLH.
When I used to stay at DLH as a kid in the 90's, they used to offer tram service to/from the DL park entrance if the monorail was closed. The only option now is to walk, unfortunately.
Even if the monorail is closed for heat, it will still open in the morning and evenings. Just midday is impacted from about 11:00 - 5:00 or so, depending on actual temps.
Some of the plans for where Avatar might go include a re-routing of the monorail beam through the Hollywood area of DCA. If this happens, some major downtime will be on the horizon in the next 5 years.