The key sentence in the section you shared is...
This is the difference between an educated noble disciple and an uneducated ordinary person, that is, when there is a place of rebirth.
The teaching gives a wider context surrounding puthujjana and aryas and what can be expected from the practice of metta regarding those two types of individuals.
Practicing metta correctly leads to rebirth in brahma realm, not to hell.
What propels that brahma realm being to hell and animal realm after death, is not the practice of metta, but their previous karma.
The sutta is not describing an absolute rule, it's just citing one possibility.
For example, other suttas show beings who perform enormous meritorious ...
A person has five spiritual faculties
All beings have these faculties and they are in a constant flux as to their level of development.
When a person develops thoughts of good-will & fully penetrates that theme with discernment, his wisdom faculty ...
Uncountable karma in even a minute, unstoppable and waiting forever for effecting as the resultant. Even they vanished immediately when arising. Their effects still possible to effect in the future proper situation.
In the dependent origination, there are uncountable past lives' intentions, Sangkhara, which still waiting to give their resultants.
There are ...
the birth in brahma realm is the result of the metta meditation. being there exhausts their positive force and since very typically they are not inclined to practice while there. then their circumstances are ripe for them to easily be swept up by their remaining unskilful tendencies and to which they have become deeply unaccustomed to ie. theyre easy prey ...