Someone who is transgendered may avoid presenting as the gender they identify as around strangers or family members in order to protect themselves or because they simply aren't ready to yet. You can be transgendered on a spectrum from only having done it mentally to full top and bottom surgery w/ hormones.
It's entirely possible for someone trans to be comfortable being open about it on the internet, but sandbox that identity to being only on the internet while presenting as their birth gender with their family and friends IRL. Telling the family/IRL friends that they're trans is still outing them.
In much the same way, if a gay person openly acknowledges their orientation online but keeps their family in the dark, you're still outing them if you tell their parents they are gay.