You're right, the benchmark comparison should be a midcap index fund.
I read the morningstar headline "This Stock Fund Is Trouncing the Market.." and the first part of the article "... fund has beaten the market" and "For the past two decades, Hennessy Cornerstone Mid Cap 30 Fund HFMDX has posted returns far outpacing the stock market" and "compared with 2.2% for its category, small-value funds, and 10.5% for the overall stock market as measured by the Morningstar US Market Index."

so I thought the appropriate comparison would be a whole market index fund like VTI or SPY.

But that article still fails and is misleading. Look at this backtest, inception to Jan 1 2024. testfol.io: Portfolio Backtester for ETFs and Asset Allocation

For most of the history, HFMDX has lagged behind its index (FSMDX) and the whole market index (VTI).
2011 to 2016 all three were virtually identical.
2016 to 2024 HFMDX lagged a lot.
YTD 2024 it finally pulled ahead.

Of course, if you had started at the low in April 2020 it beat them. You would have just had to willingly invest in a fund that lost 45% in just THREE WEEKS.

For example the last ten years a midcap index fund like FSMDX has returned 9.33% which HFMDX easily beats.

But it hasn't beat VTI over the last 10 years. CAGR 12.05% vs. 12.15%. With much higher volatility and MaxDD.
It has beat VTI over the last 5 years. But that brings to mind Jim's (mungofitch) caution about recency bias and observing just a lucky few years.

Good discussion, it's always useful to investigate potential investments & strategies. Finding something to avoid is valuable.
Nobody like a skunk at a picnic, though.