Their website doesn't list a date so I have no idea how up to date this is, but the situation doesn't seem all that clear to me: https://jpegxl.io/articles/rans/
If Microsoft's patent goes through and ends up being enforceable then their open-source release doesn't mean squat; their license may say it's royalty free and grants free usage of any patents, but the patents wouldn't be theirs to grant the usage of.
Of course you can ignore this for software distributed in jurisdictions without silly stuff like software patent enforcement, but if you distribute the software to the USA then you might run into trouble.
If Microsoft's patent goes through and ends up being enforceable then their open-source release doesn't mean squat; their license may say it's royalty free and grants free usage of any patents, but the patents wouldn't be theirs to grant the usage of.
Of course you can ignore this for software distributed in jurisdictions without silly stuff like software patent enforcement, but if you distribute the software to the USA then you might run into trouble.