If the Freenode #foo owners have a legitimate claim (ie they represent a project or community named foo) then they can register that project/community [0] and claim #foo. In case of multiple claims it is up to the Libera operators to find a solution.
While there are no technical restrictions to registering unclaimed channels, on Libera (and also on Freenode before) channels with a single # are not really free for all and on-topic projects and communities have priority.
