Soft usually means pure, deionised, or just purer.Rainwater is soft and the usual defintion is that soap lathers easily in it.How this translates is that rainwater is known to be much better and kinder for washing hair, so the statement, you wouldn't wash up with cold water so why use it on windows, Could be analgous to, You'd choose to wash your hair in the purest softest rainwater so why not your carpets?
Hot pure water tends to have more carbon dioxide dissolved in it, which is also called carbonic acid.This could affect Ph.There was a wfp company that was regularly quoted on here that said hot was dangerous because of this and must never be used.They now flog hotboxes and say hot is the best thing since sliced bread.So you are in an area where different experts say different things.