You’re right. Technically speaking matched betting/promo bagging isn’t really risk free. Not it the true sense of the term, especially compared to something like arbing, which is a risk free profit regardless of the result.
To me matched betting is a technique where good promos allow you to increase the odds in your favour so that over time, you’ll come out ahead in the long run.
Try and think of it like a coin toss. If someone was to offer you $1.50 if your selection of heads or tails came up, well you might get lucky in the short term, but over time, the expected odds compared to odds being offered will mean you’ll lose. That’s betting without promos.
But if someone was to offer you $3 if your selection came up, in this case maybe you might have some early losses, but the 3-1 return far outweighs the expected odds of 2-1, so you should come out ahead in the long run. That’s betting with promos.
So each individual bet is not risk free, but as long as you have enough capital to overcome any periods of loss, especially at the start while building a bank roll, then I’d almost guarantee you’ll come out ahead in the not too distant future.