Ah, ok I saw this table:
https://i.imgur.com/5ofzjC3.png and that trying to download required an account (which is not true for the GCC toolset) and assumed they'd make you pay something. But maybe they just try to trick you into thinking you might have to... (or I am easily fooled).
edit: My legalese is not great, but isn't the the license saying that you can't legally distribute software compiled with the LLVM-based ARM tools if you obtained it for free? Obviously the gcc-based tools can't have such restrictions.
3.2 NON-COMMERCIAL USE AND FREE OF CHARGE LICENSES:
...
(b) if you are receiving a Non-Commercial Use License or version (as applicable) of the Arm Tools:
(i) you and your Permitted Users may use the Arm Tools for internal use only; and
(ii) you are not permitted to distribute or sub-license (A) any part of the Arm Tools, or (B) Your Software,
Your Hardware, or Your Reports developed under this License using the Arm Tools. The Arm Tools shall be
used only by you and your Permitted Users, and you shall not (except as otherwise authorised in writing by Arm)
allow any other third party whatsoever to use the Arm Tools.
For the avoidance of doubt, if you are receiving a Non-Commercial Use License and the license is provided to you free of
charge, the restrictions in both sub-clauses (a) and (b) above will apply to your use of the Arm Tools.
additional edit:
Here's what happens when I try to run armclang:
$ armclang
armclang: error: Failed to check out a license.
The license file could not be found. Check that ARMLMD_LICENSE_FILE is set correctly.
I did not bother to figure out if there's a way to get a license without paying (though I would likely qualify for a non-commercial license as an academic user, but it seems like a pain).