A meta-analysis review of 36 interventional trials with nearly 6,000 participants found that the combination of aerobic and resistance training provides the greatest cognitive benefit, especially in individuals age 65 or older and those with existing cognitive impairment.
"a study that was a review of 36 interventional trials where they put people on these programs, and it was close to 6,000 individuals. And their conclusion was: is it aerobics, is it resistance training? Their conclusion was it is both. It's the combination that seems to work best... And what Jiang and the other researchers in that report noticed was that the best bang for the buck, in other words the best response, was in individuals who were age 65 or older—you and me—and also those individuals who already had some cognitive impairment." (said at 0:41:30)
A specific meta-analysis by Jiang et al. encompassing exactly 36 interventional trials and nearly 6,000 participants evaluating the combined effects of aerobic and resistance exercise across age groups and baseline cognitive status could not be located in the search. This does not prove the claim or publication does not exist, but no matching published record was verified.