OK, let's start from the top to understand International transit a little better.
First, a submarine cable is not equal to internet. A transit provider is an Internet provider. We don't all plug into these submarine cables and boom, get speedy internet. Networks are a bit more complicated than that; they're a product of peering (letting other networks know about yours) and just how much peering you do. In fact, two networks can use the same cable and get vastly different route profiles.
There are a few big cable systems between the EU, Asia and SA:
https://manypossibilities.net/african-undersea-cables/
Most of these cables are parts of large consortiums (they have many stakeholders), Seacom being the only exception and privately owned. This leads to the Seacom business model - which we'll get to in a minute.
Traditionally, these cable systems land and the wholesale transit providers buy up L2 capacity on them. This isn't internet capacity, rather L2 point to point (eg. Ysterfontein to London) capacity along a cable route - just over a very long distance. The wholesale transit provider heads to the other end of the link and drops some routing equipment down to build a POP. At these POPs, they then establish peering (a free/paid exchange of traffic) with other networks from around the world, and usually purchase onward traffic from a larger tier 1 provider. To be tier 1 means that you've done this exercise extensively and you peer (freely) in almost all peering locations in the world - operating your own core network comprised of pops and many, high capacity L2 links between them.
The Seacom model is slightly different, they sell point to point capacity to large wholesalers who want to buy it, but also operate their own wholesale transit business. Their wholesale transit business breaks out from the cable in a few EU locations (Marseilles, London, Frankfurt amongst others). But this doesn't mean that Seacom are the only tenants on their own cable. Seacom also purchases redundant capacity on WACS if I'm correct, providing dual submarine routes to their own transit clients in SA and Africa.
Wholesale transit providers worth their salt must then purchase sufficient, redundant capacity over multiple cable systems to their routers sitting all over the world. The more routers they have, the more control they have over the L2 links between them and their onward capacity. ISPs then advertise their networks to these transit providers, who in turn let the rest of the world know that their (the ISP's) networks are available through a specific provider. This is the magic of BGP. So for example, our wholesale transit providers give us access to EU, ASIA and onward routes via WACS, SEACOM, EASSY, SAT3 and SAFE submarine cables to ensure we're protected against cable breaks. We utilise more than one transit provider in case something goes wrong on the provider's internal routing network and also to provide more options to other networks to find us.
This being said, an ISP can also purchase bulk L2 capacity between SA and the EU and establish all their peering and upstream relationships from there. Some do this.
To understand how it all works: Take a server, perhaps World of Tanks sitting in an Equinix facility in Chicago. WOT uses a provider on the NTT tier 1 wholesale network as their upstream and peers at Equinix Exchange, so two possible (probably more) routes exist to get to SA. To get to us, two options exist: 1. H.E peers in all Equinix facilities, so they pick up this traffic on a local exchange in Chicago, transit it down to SA and hands it to us. (here they have full control between Chicago and JHB). 2. Our second transit provider peers with NTT in Europe. NTT carries the traffic on behalf of their client (WOT) from Chicago to a peering point in London, hands it over to our transit provider there who hands it to us in SA.
So to sum up, transit providers and cable systems are not one and the same (even though one has a confusing business model). A single transit provider usually has more than one submarine cable at its disposal, sometimes even more than just two. An ISP can put their eggs in one basket and rely on a single transit provider to let the world know they're there - but if something goes wrong on that larger network, they're invisible to the rest of the world. Advertising over multiple transit providers means that other networks have more than one route to an ISPs network.
