It'd be foolish to think a shinobi as legendary as Madara wouldn't know a fuuin jutsu.
itd be foolish to assume he does with no proof. being "legendary" doesnt mean youve branched yourself into every single type of ninjutsu, especially one that in madaras era was specialised in by mito's clan. why would he involve himself in a clan whos allied to the senju who he hates?
also there are many kinds of seals. theres the binding seal from danzo, soul sealing, object sealing, jutsu sealing, scroll seals etc. madrara has shown NONE of this.
by your logic, tsunade is a legendary nin so she must know genjutsu.
If he wants to become the juubi then he has sealing jutsu and as strong as Sage.
i think most people care about his ability to stop edos not juubi