Obviously people vary, but I think 10 minutes would be quite generous even if passengers were walking from the back of the mainland train, or were slow to disembark the boat.
The transfer to the ferry at both Harbour and Pier Head is much shorter and easier than getting to the tube at a London terminus. It's level, and can't be much more than 100m between the front of the train and the boat embarkation ramp.
In the other direction there is a gentle up ramp from the ferry towards the train, but the distance is even shorter, especially at Harbour, as it short-cuts the loop where the ferry boarding queue is held.
I honestly can't see how anyone could come up with the 19 or 20 minutes quoted above as the journey planner minimum times.
In London I regularly cross between Euston or KXSP and Waterloo, and I can usually do those in around 20-25 minutes if the tube is running normally. Someone slow on their feet or with luggage (or with tube delays) might be nearer the 40 minutes though. What does annoy me is the 75 minutes or so that's often in the itinerary when I have an advance ticket with compulsory counted-place reservations on SWR, meaning specified train only. The Waterloo Tap is often the beneficiary

.