Based on the talk [0] where the OP suggested it number of times. She mentions the engineer being rusty and having significantly fewer code contributions than her peers.

In general, if the glue person has intermediate knowledge of the codebase, they'd get promoted without much controversy. In such a case, it should not be difficult for them to bang out a couple of features and ship it, for example.

[0] https://www.youtube.com/watch?v=KClAPipnKqw