I haven't seen that rubber ever in a reco kit.
Generally, it would last the life and beyond of the master cylinder. (imo)
I didn't see the pic of the rubber on the website, maybe a direct link would help.
Basically, if the rubber you're after is shown as 'a part' of 'their' service kit, then they are under obligation to supply it.
Lord of the Bings, kangaroo farmer