Diabled Persons Railcard - discrimination?

I have been diagnosed with cancer (NHL), which is classed as a disability.

https://www.cancerresearchuk.org/about-cancer/coping/practically/the-disability-discrimination-act-equality-act-and-cancer

I attempted to get a Disabled Persons Railcard but they said no as I do not meet their eligibility criteria.

.https://www.disabledpersons-railcard.co.uk/are-you-eligible/

This is surely a form of discrimination?