[Dirk Vdm noted:]
The light signal has velocity c w.r.t. the embankment.

[Cadwgan replies:]
Show on paper explicitly how this happened.

[Dirk Vdm noted:]
But Einstein was talking to physicists, not to laymen.

[Cadwgan replies:]
Nope, his book specifically addressed the educated layman.
(See the preface.)