Many players reported being joined by Helldivers with brightly colored names with the ability to summon weapons out of thin air who represented themselves as developers testing out features. However, if you have stumbled upon a player acting like that, they were misrepresenting themself. Helldivers 2 Community Manager addressed these rumors and issued an important PSA about these devs.
Source: N4G PC A CM Issues a PSA About Players Pretending to Be Devs in Helldivers 2