I think using the word irreversible for both outcomes is entirely a matter of perspective. A person choosing to abort can still be a mother which would technically mean that option is highly reversible. Really just depends on what you consider the start and end states before and after either option is done.
There are a number of reasons a person might not be able to get pregnant again after an abortion. Abortion always carries a risk of sterilization. Other life factors such as age or health can also mean that a pregnancy can't happen again.
