We may have two processes at work here. I buy the thinking that roasted and ground coffee is of a texture that it can absorb lots of molecules. The grounds, provided they’re not so small that they’re powdery, have ample space between the grains that there’s a good chance that coffee can function as an absorbent. But there’s also the series of anecdotes that attest to its power to deal with garlic and other substances. I have a different version - when you’ve eaten garlicy food and you want to lose that taste, you can, if you’re tough, chew one or two roasted coffee beans. It de-garlics your mouth. (Of course, you then have to rinse the coffee out, but the problem is solved by that time.) So I think there may be some chemical affinity that coffee, itself, has for some esters or some other pungent molecules, too.
Once, my husband tried to make microwave popcorn, but he set the microwave for 5 minutes and walked away.
After we cleared the vast cloud of black smoke out of the house and sent the firefighters away, my poor microwave stank. I tried vinegar, baking soda, lemon juice, and finally, coffee grounds worked to get that stench out. I left the coffee grounds in the microwave for about 4 days, and it didn’t stink anymore. I can’t explain the science, but it worked.