I would have thought the main difference is that in cases of abnormally early puberty is that the girls come off the blockers after a couple of years and go on to experience a normal puberty.
For trans children the outcome most likely is that they go straight from blockers to cross hormones and never experience a natural puberty. It sets them down a path that leads to infertility and life long medical intervention.
Proper research has to be done into the risks associated with never going through puberty. We have already seen some evidence that it affects the brain, maturation and intelligence. This needs to be looked into further.