Old English descended from some other language (the name of which does not matter for the purpose of this comment).
Just because some accident caused the name "Brythonic" to be applied to languages older than the oldest language with the name "English" says nothing about the importance or value of Brythonic relative to English. The assigning of names to no-longer-spoken languages is arbitrary enough that you cannot use the names to prove anything about reality.
Just because some accident caused the name "Brythonic" to be applied to languages older than the oldest language with the name "English" says nothing about the importance or value of Brythonic relative to English. The assigning of names to no-longer-spoken languages is arbitrary enough that you cannot use the names to prove anything about reality.