This answer doesn't really depend on "magic", but on the idea that the universe in which your story unfolds is somewhat similar to a simulation. However, it does not require that the fact that demons can be controlled by knowing their True Name is a purposefull, conscious decision by the creators of the simulation, so I don't consider it a "because [the gods] said so" explanation.

#Setting The ur-language could be the instruction set of the simulation. In this simulation, entities interact by sending messages to each other and to The Simulation itself. For instance, to lift a bucket, a person's 'handle' in the simulation would inform The Simulation that a force of, say, 1 N should be applied to the 'handle' of the bucket, in a 90 degrees angle to the ground.

We can represent this with the following ASCII-art:

<Person> --[apply-force: 1 N, surface-angle: 90 degrees]--> <Bucket>

#The source of the demons' power Demons, or really any supernatural element in the simulation, are entities which figured out a way to install proxies in their interaction with others. The demon can check which messages are sent to its proxy, and modify the message before it is sent to itself. When a person interacts with your Glabrezu, instead of this happening:

<Person> --[push-over]--> <Demon>,

this happens:

<Person> --[push-over]--> <Demon-proxy> --[do-nothing-in-particular]--> <Demon>

This way, all the supernatural things you describe are possible: the proxy can report the Lamashtu is not visible when in fact it should be; it can claim to have arbitrary amounts of energy available, thereby being able to reach arbitrary speeds up to the speed of light (sorry, darkness); absurdly large Glabrezus' proxies can claim the Glabrezu has a dexterity it should never possess, etc.

#True Names as a way to subvert that power True Names, now, are references to the true identity. By knowing a demon's True Name, a person can bypass the proxy:

      '----[push-him-over-anyway]----^

With the proxy bypassed, all 'supernatural powers' of the demon are out of the picture, and the demons have to face the harsh reality that their physique is rather unfit for real life. At this point, even a child could force them into submission.

This answer doesn't really depend on "magic", but on the idea that the universe in which your story unfolds is somewhat similar to a simulation. However, it does not require that the fact that demons can be controlled by knowing their True Name is a purposeful, conscious decision by the creators of the simulation, so I don't consider it a "because [the gods] said so" explanation.

#Setting The ur-language could be the instruction set of the simulation. In this simulation, entities interact by sending messages to each other and to The Simulation itself. For instance, to lift a bucket, a person's 'handle' in the simulation would inform The Simulation that a force of, say, 1 N should be applied to the 'handle' of the bucket, in a 90 degrees angle to the ground.

We can represent this with the following ASCII-art:

<Person> --[apply-force: 1 N, surface-angle: 90 degrees]--> <Bucket>

#The source of the demons' power Demons, or really any supernatural element in the simulation, are entities which figured out a way to install proxies in their interaction with others. The demon can check which messages are sent to its proxy, and modify the message before it is sent to itself. When a person interacts with your Glabrezu, instead of this happening:

<Person> --[push-over]--> <Demon>,

this happens:

<Person> --[push-over]--> <Demon-proxy> --[do-nothing-in-particular]--> <Demon>

This way, all the supernatural things you describe are possible: the proxy can report the Lamashtu is not visible when in fact it should be; it can claim to have arbitrary amounts of energy available, thereby being able to reach arbitrary speeds up to the speed of light (sorry, darkness); absurdly large Glabrezus' proxies can claim the Glabrezu has a dexterity it should never possess, etc.

#True Names as a way to subvert that power True Names, now, are references to the true identity. By knowing a demon's True Name, a person can bypass the proxy:

      '----[push-him-over-anyway]----^

With the proxy bypassed, all 'supernatural powers' of the demon are out of the picture, and the demons have to face the harsh reality that their physique is rather unfit for real life. At this point, even a child could force them into submission.