This is incorrect - they can split however the driving chip supports. (Q|O)SFP(28|56|112|+) can all be split to a single differential lane. All (Q|O)SFP(28|56|112|+) does is provide basically direct, high quality links to whatever you chips SERDES interfaces can do. It doesn't even have to be ethernet/IB data - I have a SFP module that has a SATA port lol.

There's also splitting at the module level, for example I have a PCIe card that is actually a fully self hosted 6 port 100GB switch with it's own onboard Atom management processor. The card only has 2 MPO fiber connectors - but each has 12 fibers, which each can carry 25Gbps. You need a special fiber breakout cable but you can mix anywhere between 6 100GbE ports and 24 25Gbe ports.

https://www.silicom-usa.com/pr/server-adapters/switch-on-nic...