This is pretty much boilerplate legalese that's generally intended to mean "you can't upload a file to your public folder, then sue us for copyright infringement because that file is in a public folder".
Lawyers seem like to cover their clients' asses as much as possible, but for a service like this that's designed for privacy-conscious people it'd do them well to step back and think about how the license comes across to privacy-conscious laypeople.
From what I've seen of Keybase, from back when they were just an alternative to the PGP Web of Trust to now, I don't think they're genuinely planning anything nefarious (but they'd still do well to have a ToS that reflects that).
While the rest of the paragraph looks standard to me, I'm not completely sure about that last sentence. Is that what you're wondering about?
If say, in 3 years from now as a provision of their service they offer analytics, or direct marketing. they can use any of your content for that.
I get this is probably just a wide net they are casting to not box themselves in, but it's something that is super restrictive to the user. And being as they also have a section require arbitration, and barring class action suits.
If they are acquired by, or decided become a bad actor, then you are pretty much SOL.
Even if it did mean that, it would be of 0 use to them because they are fully end-to-end encrypted
https://keybase.io/docs/terms
When providing Keybase or the Service with content, such as your name, username, photos, social media names, data or files, or causing content to be posted, stored or transmitted using or through the Service (“Your Content”), including but not limited to the Registration Data and any other personal identification information that you provide, you hereby grant to us a non-exclusive, worldwide, perpetual, irrevocable, royalty-free, transferable (in whole or in part), fully-paid and sublicensable right, subject to the Privacy Policy, to use, reproduce, modify, transmit, display and distribute Your Content in any media known now or developed in the future, in connection with our provision of the Service. Further, to the fullest extent permitted under applicable law, you waive your moral rights and promise not to assert such rights or any other intellectual property or publicity rights against us, our sublicensees, or our assignees.
That's a bridge too far, and someone needs to dial this back.