Hopefully this is the reason. I could understand operators on the longer crossings not wanting cyclists, because they take up cabins that they could be used by car drivers/passengers, who of course pay higher fares, but that isn't a factor on the Dover routes. If all P&O want to do is get rid of the hassle of getting people onto the ship by foot (and, as you say, fall into line with what DFDS already do) then hopefully cyclists will still be allowed. Not great for genuine foot passengers of course, but hopefully not going to affect cyclists.
I'd always favour DFDS to Dunkirk anyway for a number of reasons, but would be good still to have the option.