The company is almost certainly lying about her age.
Take a look at the 4th gen who are apparently younger than her.... or go back and look at any of the other members at that age. Tanaka Natsumi was younger than her when she joined. You could also compare to Matsui Jurina, but Jurina has always looked 4 years older than her stated age so they might be lying about her in the other direction.
Maria situation is a bit... odd. Remember that her mother was already HKT48 staff, but they decided to enter her through the Draft audition where she imaginably could have been snatched up by any of the groups. Moreover, the members constantly treat her like a very young child-- and they have never treated anyone else who was her supposed age in the same way.
Being only 130-something centimeters at 13 years old is incredibly odd, even for Japan where its not unusual for people to be "short". She is 20 centimeters shorter than even "very short" Japanese people her age.
The only other possibilities is that she has some sort of dwarfism that, lucky for her, doesn't give her the stocky body and... unusual... facial features that are typically caused by this, she just looked 3-4 years younger than she is...
Or that her health is just in absolutely horrendous shape due to poor nutrition. Obesity causes girls to go through puberty unnaturally early, so the reverse is also possible... and she is ridiculously thin as well as short. But she also seems way more energetic than someone who was starving so badly that her body couldn't grow would likely act.
However cute it might be that she is so tiny, sadly-- in terms of her long-term health and happiness, the best possible solution is that the company lied about her age because they figured it was fine to have her join her mother at work after school and do things with the older girls. So if they wanted to let her join the group at 8 years old but they said no one younger than 11 can audition, they might have felt it was necessary to just say she was 11 and not really worry about it. As long as they aren't having her do too much (and they do seem to limit her activities), its not illegal. She the only one in the group who isn't active on any SNS, she posted only a little on 755 before abruptly stopping.
Then again, the fact that there is a picture of her and Ota Aika taken several years ago, that seems pretty conclusive proof that they can't simply be lying about her age. Also, she talks about being in middle school and if she was really in 4th grade rather than 7th grade, one would imagine that someone would expose her.
Still, don't dismiss the idea that maybe she is just younger than they say so easily. It really is a simpler explanation than positing that she is the only 13 year old in all of Japan who doesn't have a medical problem who hasn't even reached 135 centimeters. But if she is actually only 9 or 10 years old? She is average height.