Immediately following the contest on Sojourn, Plagueis had commanded members of the Sun Guard to locate the starship that had allowed Venamis to infiltrate the Hunters' Moon, then move it and the poisoned Bith to Aborah. Larsh Hill and the other Muuns had been apprised that an intruder had been captured and disposed of, but no more than that. An investigation of the ship had yielded data that might have surprised even Darth Tenebrous, who had provided the ship. It seemed that well before he had confronted Plagueis or learned of his Master's fate, Venamis himself had been scouting for potential apprentices. Plagueis could not help being impressed, though begrudgingly. The young Bith would have done well in Bane's era. Now, however, he was an anachronism, and by extension, Tenebrous also.
--Darth Plagueis
You are falsely assuming this is referring to his fighting capabilities, when it is very clear from the context that Plagueis is impressed by Venamis' initiave in finding apprentices, not his fighting capabilities. So in short, Plagueis thinks he would have done well in Bane's era due to his philosophy regarding multiple apprentices, which is very much akin to the pre-Bane mentality.