The most popular definition of cladogenesis is the splitting of evolutionary lineages (cessation of gene flow),
At the risk of going wildly off topic - this touches on the reasons why the BSC has also seemed a no-starter to me, and I struggle to understand why it's still treated as if it's the default definition of species - despite it being one that is never used by taxonomists.
Cessation of gene flow can take an extraordinarily long time and, unless physical barriers (like oceans) get in the way, can sometimes take place long after populations have become clearly separated from one another.
Geladas are clearly a separate evolutionary lineage than normal baboons. Nevertheless, hamadryas baboons have been observed to occasionally mate with geladas, and genetic testing has established that this is ongoing gene flow into one of the two populations (it only goes one way, if I remember correctly, due to hybrid incompatibility depending on the sex of the parents).
The article you cite is on the right track, in my opinion. The boundaries between two species are no more meaningful than those between genera and families - they're wholly arbitrary and far too much ink and thought is wasted on them. I think arguing that creationists are wrong because speciation has occurred is a mistake. Speciation has no clear, objective definition.