Looking at the patch itself (linked in the article), the description has this:

> We now support configuring bandwidth up to ~1 Tbps (overflow in m2sm at m > 2^40).

So I think that's it, 2^40 is ~1.099 trillion

Looks like an arbitrary validation cap. By the time we're maxing out the 64-bit underlying representation we probably won't be using Ethernet any more.