MWEB already peers with MTN. Here are the two hops between their networks1:
6 g-0-3-vic-jinx-2.mweb.co.za (18.104.22.168)
7 mtnns-2.jinx.net.za (22.214.171.124)
I think what MWEB probably means by "transit" is that MTN charges them for peering. If MTN agreed to MWEB's ultimatum, they would still be peering at JINX. So if you are an MWEB customer who likes to surf Mybroadband.co.za2, you should demand a discount to compensate for the de-peering.
"We do not believe in paying for transit or selling transit. Rather, we have invited all of them to peer with us."
-- MyBB quoting MWEB ISP CEO Derek Hershaw
What makes buying transit in London different from Johannesburg?