Habermas frequently uses the adjective "weak" to qualify his use of terms like transcendentalism etc. It is not often clear what Haberams exactly means by the qualification. Here is a clue from Habermas himself (from his discussion of Putnam in TJ):
"[The] indispensability argument is "weak" insofar as it argues not that a presupposition is unrevisable or necessary, but that it is - for the time being - unavoidable, that we cannot imagine it being otherwise." (pp. 219-220).