Can someone please explain to a layperson how we know that the $B$ meson decay into a muon doesnt then decay into an electron?

I’m not specifically familiar with the recent results you refer to, but muons decay into electrons with characteristic lifetime $2.2 mumathrm s$ (in their own test frame), which corresponds to a characteristic distance they can travel before decaying.

Particles created in collider experiments tend to be moving at an appreciable fraction of the speed of light, meaning that they can get quite a long way before they decay. This is certainly true of the muon/anti muon pair produced in the decay of a $B$ meson. As a rough lower limit, the speed of light times the muon lifetime yields a characteristic decay distance of about 600 meters; the real decay distance can be much larger due to the time dilation effect, which effectively prolongs the lifetime of particles moving near the speed of light.

So to answer your question, relativistic muons don’t decay until they’ve traveled hundreds of meters away from the point at which they were created.