Some feedback from Flow github: https://github.com/vaadin/flow/issues/5040
IMO the iron icons should be extending Icon. If it is not for some reason possible, then you should create and issue / fix for that.
Thanks for the heads up! :)
I created an [issue]
(https://github.com/FlowingCode/IronIconsAddon/issues/2) in the addon repository regarding our use of IronIcon (which I understand is correct because we don’t need to bring the vaadin-icons iconset), and I also answered #5040 about using icons from the iron-icons iconsets in Vaadin (without the addon).