Coercion refers to the use of violence to obtain a desired goal.
Where enforcement of property rights is used to obtain desired goals, however, this does not count as coercion – even though enforcement involves violence by definition – because trespassing is considered a form of violence and therefore the trespasser is the initiator.
When one person has something that they don't need in order to live, and another person needs that thing in order to live but doesn't have it, marketists consider that the first person has no obligation to provide the thing in question to the other person. They describe such obligations as "positive", and argue that only "negative obligations" are ethically mandated.
Threatening to withhold life-saving items if someone doesn't do what you want is therefore not considered coercion, while threatening to punch them (even non-fatally) is.
In other words, marketists don't believe in economic coercion. They therefore consider such things as sweatshops and wage slavery to be perfectly acceptable, because coercion (in their definition) was not used to obtain that labor (leaving aside cases where workers are physically threatened). They argue that nobody is truly free unless they are free to sell themselves into slavery.