Yes. that is the exact problem. These is someone who sounds very much like Bix in the background and someone who sounds very much like Secrest in the foreground. Also, there are the unexplained fill ins earlier on in the recording .Like you, I had also assumed that Margulis was a faultless player. Oh well ! perhaps Margulis thought that he would emulate Secrest, just for fun, and Secrest decided to pull his finger out and, for once, really "do a Bix" just to fool record collectors 80 years later.