From Wikipedia: "The alt-right has no formal ideology... 'Alt-right' is a recently coined umbrella term, with no clear criteria of membership yet agreed upon."
https://en.wikipedia.org/wiki/Alt-right
Would each of the people being banned agree that they've signed on to an admittedly racist and sexist platform? I doubt it.
EDIT: Let me amend "I doubt it" to "I am not sure and haven't done enough research to have an informed opinion." From the article I can't get the full set of people that are known to have been banned or how many of them are professed white nationalists...
Twitter's move reduces their platform and reach but increases their cultural cachet — they're becoming the Certified Opposition to all of society's ills.
Even on /b/, which is known for putting up with the absolute lowest quality shitposting on the entire internet, insulting someone as a cuck will now be met with a litany of retorts regarding your "low quality bait". Weird times.
If they let some definition take hold that includes racism, then in the future when they object to saying that the alt right is racist, they aren't standing up for what they believe in anymore, they are providing cover for the racists that stole the label from them.
edit: honest question