The strictness is in what may be listed in the robots txt, not the User-Agent header as sent by bots. the example given in the linked draft standard[0] makes this abundantly clear that it's on the bot to understand how to interpret the corresponding lines of robots.txt.
Of course, in practice robots.txt tend to look less like [1] and more like [2].
[0]: https://tools.ietf.org/html/draft-rep-wg-topic#section-2.2.1