> A splitting timeline would need a definition of an option, a decision.

This is a reasonable conclusion to reach from the informal description you were given, but (I'd argue) not a valid objection to MWI itself.

Mainstream theories of quantum mechanics share the idea of entanglement and superposition. Think of Schrödinger's cat experiment where a radioactive atom gets in a superposition of "decayed" plus "not decayed", then interacts with the detector so there's a system in the superposition of "decayed & detected" plus "not decayed & not detected", and eventually a superposition of "decayed & detected & cat dead" plus "not decayed & not detected & cat alive".

Copenhagen interpretation says this stops when it interacts with a "classical" observer, which is left undefined, and collapses into one of the possibilities at random. Wigner interpretation says similar, but defines observer as being a consciousness somewhere between a mouse and a dog.

Many-worlds interpretation says there are no "observers" and the whole universe is a quantum system. Consequence of this is that entanglement "bubbles up" until the entire universe is in a superposition. There's no definition of "choices" or even "worlds" being relied on.