Is it ok to let a job that is still eyeing me know that I already accepted another offer but I'm still open to theirs?

They said after taking a look at my art test they will get back to me. They offered more, so I am worried if I let them know I already accepted another offer if I will look unreliable? I can't keep telling them "yeah my hiring manager said he will give me more time to decide", that looks so sus if I keep saying that, and I already told them the other manager wants a response so that they can expediate the process.

I don't want to look unreliable so I'm not sure if this is an intelligent thing to mention that I already took the other offer if they ask.

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