Not necessarily, depends on experience with that one. An older lead worker is more likely to boss it instead of pulling all his gear up for a small job. I'm aware due to being a leadworker.
My city and guilds training course was a 5 day one and we certainly didn't use code 4 for a hopper.
11 is maybe too low for slates unfortunately, you can use this https://www.permavent.co.uk/products/easy-slate/ which can go to 12 or 10 with cement fibre slates
So you've used the wrong term for years and just going to go with it when others tell you you are wrong, cool.
I'll let all the roofers working with me and the lead workers that I get in know they are wrong.
https://www.amazon.co.uk/2018-PROGUTTER-Gutter-Cleaning-Scrapers/dp/B078YDMMWG/ref=sr_1_6?crid=KKVYYF82HI2U&keywords=pro+gutter+cleaning+tools&qid=1666637517&qu=eyJxc2MiOiI0LjIzIiwicXNhIjoiMy4zMSIsInFzcCI6IjIuOTAifQ%3D%3D&sprefix=pro+gutt%2Caps%2C113&sr=8-6
That with an extension pole will do...
Your slates are fine, they are just a cheaper quality slate. The rust will happen now, nothing you can do except replace them.
The lead is an issue, whatever they have used is reacting with it badly.