What do you mean by "the way he trained"? Do you mean the way Mentzer trained himself during his competitive days back in the 70s? If so, then he didn't train in the HST fashion, but he did train 4 days per week and split the body in half training each muscle twice per week. Anyways, who gives a shit what others say, the only way to know for sure is to try it yourself
He actually split the body in half training body parts either twice or once a week. This was how he trained for the 1980 Olympia. Monday would be legs, chest and tricep. Wednesday would be back, delt and bicep. Friday would be Monday's routine. Next week Monday would be back, delt, and bicep. Wednesday would go back to leg, chest, and tricep. Friday back to back, delt and bicep. Mr. Florida Frank Calta trained in a similar way and he called it rotation for recuperation.
He did win the IFBB Mr. America using a whole body routine. He then went to a 4 day split training half the body at a time. His last competitive routine was the three day a week I described. The routines he recommended after he retired I don't believe he ever used when he was involved in contests.