Legionella’s disease is probably what the someone was on about. Have a quick scan of the wiki article if you want. Any water that is left in a system and does nothing “may” become an issue. If the system was drained down before hibernation and crucially refilled BEFORE powering up you would be in safer territory immediately. Whoever definitively answers this question for you should have no problem with you asking for their qualification and experience in the water safety field.
Yeah, Wikipedia says 70°C hot water kills them practically instantly: https://en.wikipedia.org/wiki/Legionella#Legionella_control_and_biomonitoring
So, I guess, if you heat it up beyond 70°C and leave it running for a minute before you step under the shower, that would presumably be safe, but I'm not an expert either...
This is about as definitive as you’ll get online:
https://www.hse.gov.uk/healthservices/legionella.htm
It’s water, after all. Regardless of where you live in the world.
Pretty sure UK law is/was: hot water stored at 60+, distributed (i.e. out of the tap) at 50+. Cold should be stored and distributed under 20. It’s astonishingly common in the UK for hospitals, hotels, care homes etc to have a tap-running check list where an individual runs unused taps for a set time (30 seconds?) every so often. In my limited experience your shower head is the badboy you need to pay most attention to - but that’s purely anecdotal.
All on science and research about water