From the NHS web site.
"The two things that Legionella bacteria need to grow and reproduce are:
a water temperature of 20-45C (68-113F)
impurities in the water that the bacteria can use for food – such as rust, algae and limescale"
(It also says "Legionella bacteria is usually found (often in harmlessly low numbers) in sources of water, such as ponds, rivers and lakes. However, the bacteria can rapidly multiply if they find their way into artificial water supply systems, such as air conditioning systems.
Large buildings, such as hotels, hospitals, museums and office blocks, are more vulnerable to Legionella contamination because they have larger, more complex water supply systems in which the bacteria can quickly spread.")
So - if there are no impurities such as rust, algae or limescale in a window cleaner's pure water system = no chance of legionella whatever the temperature.
If there are impurities then the temperature must also be within the given range for long enough.
I shan't worry, but thanks for the warning - if were to run a fleet of vans and backpacks with possible careless operation standards then I might fiddle about and do something.
As a sole operator with storage tanks in a darkened shed and an insulated van tank in a windowless van I will do nothing.
But I would drain a backpack and clean it through if it had been stood for say a couple of weeks unused.