So... Yes, they did talk to me about their feelings of imposter syndrome, out of a recommendation from someone else (I frequently mentor people at work). Unfortunately this person demonstrates a cluster of traits that means that they cannot acknowledge when they actually aren't good at something. (This person has a history of accomplished parents and education/career "boosts" from diversity goals (I am fine with diversity programs giving a boost to folks who otherwise are passed over, in fact I have benefited from them and I am grateful- the issue in this case is that this person actually isn't talented in this skill area)). I also know from others that this person's work performance is consistently, significantly below expectations in terms of measureable output and achievement of routine tasks that all team members should be able to do. Put these two together and their explanation for their feelings of inadequacy is latching on to the explanation of imposter syndrome. It's not my job nor my place to tell them that this isn't the case (I don't manage them), so at this point, I just try to avoid this person (largely due to the aforementioned cluster of traits).

They probably just suck at their job.

That’s not imposter syndrome, that’s just sucking at your job and not getting fired or laid off yet.