Derailleur rattling?

Guy brings me a bike to check out. A Specialized Sirrus from about 2014 8 speed DT. Says the derailleur was rattling and shifting on its own at times. I put in on the stand and it seemed to shift ok really/ He need the back wheel trued also and was really a wobble.
I ask him if he did anything and he said he did go home and clean and lubed the chain. The chain look pretty clean too. So I managed to get the wheel trued and it is pretty decent not like truing a precise road machine with expensive wheels but it is fine.
I then cleaned the chain good again and adjust the back V brakes. Was a bit off but now they are even on both side. I lubed some point of contact in the brakes at the frame and then clean the rear derailleur. I then drop some lube at the pivots on the RD.
I made absolutely no adjustment to the RD. I did make a very small FD adjustment to take out some rub on the big cog in front in the middle ring when in the small rear.
Right now on the stand the bike shifts perfectly in all combinations and quite good I might add. So do you think what this guts problem was all along a dirty non-lubed chain? I know if things not lubed it will shift funny is ghost shifting a product of dry chain? In the end ghost shifting suggest to me much more problems but frankly I had this back working in 20 minutes.
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Deacon Mark