From the above mentioned article.

"It is always more expensive to lay a horse than to back it, this has to be otherwise the book would not add up. So the lay price of a favourite at traditional 3/1 will be around 4.7 decimal..

If you lay this horse to lose for £1 at 4.7 and it does lose, you stand to make £1 minus Betfair commission of 5% so you get your £1 stake back and a profit of 95p. All very good.

However, if perchance the contenders do not perform, your horse suddenly wakes up and goes on to win, then you have to pay for every other horse in the race that lost. So your loss would be your £1 stake* 4.7 = £4.70"

How can a so called professional get this basic concept so wrong???
Your loss is only 3.70.