I wouldn't worry too much about the ESA50 going astray. I haven't heard of that often happening.
If you are concerned you can occasionally phone to find out if the form has been sent. If the form did get sent out and you didn't receive it you would be entitled to ask for more time to complete it.
That has put my mind at rest because I didn't know exactly what the DWP guy meant either.
It was only after the call had finished I started to think about what had been said. And then allsorts of things started running through my mind and I've been worried since.
If it doesn't arrive in the next couple of days or so, I think I will phone and ask them if it has been sent.