To “back” is to place a bet where you stake a certain amount, and win if the specified outcome DOES happen. To “lay” is simply to take the other side of that bet - you receive the backer’s stake, which you keep if the specified outcome DOES NOT happen, but you must pay out the backer’s winnings if the specified outcome DOES happen.

In something with only two possible complementary outcomes “Trump wins” and “Trump loses”, to LAY the outcome “Trump wins” is equivalent to BACKING the complementary outcome “Trump loses”.

The implied probabilities 21.7% and 20.8% do not refer to the complementary outcomes “Trump wins” and “Trump loses”. They both refer to the “Trump wins” outcome, and they are the bid/ask spread for that outcome - analogous to the bid/ask on a stock being (say) 50 1/8 - 50 1/4, where you receive the slightly lower price if you want to sell, you pay the slightly higher price if you want to buy. In this case, with respect to the outcome “Trump wins”, if you want to BACK that outcome there is someone in the market who will take the other side of your bet at implied probability 21.7%; whereas if you want to LAY that outcome, there is someone in the market who will take the other side of your bet at implied probability 20.8%.

And again, LAYING “Trump wins” at an implied probability of 20.8% is completely equivalent to BACKING the complementary outcome “Trump loses” at an implied probability of (1-20.8%)=79.2%.

If you want to consider the market entirely in terms of what outcomes you “back”, the market is:

you can back Trump to win at implied probability 21.7%

you can back Trump to lose at implied probability 79.2%

Considering the market this way, you will always find that the sum of the implied probabilities of all possible outcomes is slightly more than 100%, which here represents the bid-ask spread in the marketplace (equivalent in a historical context to the bookmaker’s vigorish).