That is incorrect. Worm universe is built on something like "quantum multiverse" model. Each event with multiple possible outcomes splits world in several worlds. There were such splits long before scion arrival (low-tech worlds where Cauldron got their cases 53 in canon, barren planet which became Avalon in this fic), there were such splits after his arrival (Dalet). Amelia and Atonement could be different universes in that multiverse.
Taylor was born more than a decade after Scion arrival, so it can be easuily explained as butterflies. I can't remember differences between Aleph and Bet that predate Scion.
Unless I totally misunderstood Worm multiverse it wasn't build on the quantum multiverse model, it was built on the idea of a finite but huge multiverse, all the universes began with the big bang each being entirely identical, but due to Chaos theory some universes has developed differently from others, and the further you go from the start of all the universes, the more universes diverge from each other.
The number of universes are however so huge that there still exist a huge amount of planets that are near identical, earth bet and earth Aleph is near identical, since the same events have happened there.
But there are probably huge differences, on a galactic or universal scale, since the fact that the earth developed the same don't mean all the other planets in their universes did.
The Entities however are each unique, since they spread to all iterations of their origin world, before they destroyed it.
So while at the start of the Entities evolution, prior to them becoming interdimensional, there was probably billions and billions of different versions of the Entities, when they began to spread they began to interact, and therefore each became unique, this became even moreso true after they left their world, since they mark a route as already used, so other Entities won't come.
If universes split endlessly the entities wouldn't have a any need for the cyclesm since they could continually make a bigger and bigger universe by combining universesm thereby prolonging heat death.
And since the new bigger longer lasting universe would also split, there would never come a point where all available universes for combination was already experiencing heat death.
So yeah Worm multiverse don't split.