IJR
iShares Core S&P Small-Cap ETF
stock NYSE ETF

You would absolutely use small and mid caps for diversification. There are a couple that I'm actually very fond of. But you would rarely pick any for growth. And that was the scope of the post if you read it. I can point out many funds that beat A,B,C and X,Y,Z but it doesn't mean it fits for every person and every purpose. I would never use IJR as more than a satellite, but I would use QQQM as a core. 

IJR outperformed QQQ over the last ~25 years.
https://imgur.com/a/ijr-vs-qqq-2000-to-2025-7OOKO5j

My longest term ETF, IJR since 12/2003 abg annualized return 15.209%.
Best performing ETFs
(ROTH) VGT 4/16' 743.99% or 72.178% annulized.
(Brokerage) QQQ 2/27' : 449.09% or 49.57% annualized.