https://www.dw.com/en/north-sea-german-m...scue/a-44677718
The article in German states that the three people were surprpised by the flood.
Tidal tables are available for this region (and should be used to plan walks). The tidal amplitude makes the water rise too fast to outrun it - especially in the mudflats. A good whistle, PFD and maybe a Cyalume light might come in handy when one gets in a situation like that.
Tourists regularly get in trouble there. Once you see the water rise you already are in trouble.