Wrong interpretation. Savin had a very good record and was never stopped coming into the Jackson match. In addition, the two hated each other. Jackson vs. Slavin was a brutal fight for the British Empire title, which back then was a big thing. It is a USA interpretation that Johnson was the first Black Heavyweight Champion, you could argue Jackson was and be correct too. Mitchel never KOd Slavin. Post Peter Jackson, Slavin side a bit, but still beat name guys like Frank Craig, Joe Butler, and Tom Lees.
The press speaks of Jackson's skill. He has a quick 1-2. Tall and 195-200 pounds, Jackson was a big man for his time. George Siler who saw them all up until his death said Jackson was better than Jack Johnson by long odds.