If water damages units then the units are not fit for purpose mate, it's like saying this umbrella is the best you can buy just don't get it wet......
Seriously can't see any reason why pure water can affect a window. All that lead is basically the same stuff, sticks to the pane and is rolled on with a hand roller. I can see that if you hammer the window like a mugger on your mum you might be able to lift a piece on an old window but you'd have to be extremely unlucky and the lead would have to be really crap.
When you make units and black the lead you use 000 wool and T cut, well we used to. Then you polish it up with a rag and I can assure you I was never light handed buffing them up and I've never shifted a piece of lead with a rag in hand.
Units break down due to a blown seal, what causes the seal to blow is always up for debate, mainly bad fitting or a really crap product used to hot melt the two panes together.
If someone told me to my face that pure water and a brush could ruin 20k of leaded windows I'd tell the mug to take em back out and find another fool
Rather annoying that firms say things like this and stupid really as they are only rubbishing their own products.