T-SQL Purpose of MAX in this Group By Query
You say: "My best educated guess is that somehow max is being used to avoid multiple grouping columns"

That is correct. 

and then: "... but how can this return the correct results?"

It returns correct results because the Symbol is the primary key in both the Investments and the Price tables. Therefore, any aggregate function over a P.column or an I.column is aggregating identical values. And MAX(c) when c is 2, 2, 2 or 2 is of course 2. 

Could the query be written somehow else, possibly without all these aggregations? Yes, see a related question: Why do wildcards in GROUP BY statements not work?

It would have to be a rather long GROUP BY clause or have the aggregations moved into a subquery with only the Holdings_Secure table (where Symbol is not the Primary key) and then joined to the other two:

SELECT
    I.Symbol Symbol
,   I.Ticker CUSIP
,   I.Name Name

,   H.TotalQuantity
,   H.TotalMarketValue
,   H.Price

,   I.CategoryCode5 BUY_SELL
,   I.EquivFactor1 PriceTgt
,   P.LastPrice CurrPrice
,   I.AssetClass Target
,   I.Industry Industry
,   I.CategoryCode1 Risk
FROM
    ( SELECT 
          SUM(Quantity) TotalQuantity
      ,   SUM(MarketValue) TotalMarketValue
      ,   MAX(PriceLC) Price
      ,   Symbol
      FROM
          HOLDINGS_SECURE
      WHERE
          Quantity &gt; 0
      GROUP BY
          Symbol
    ) H
  JOIN
    INVESTMENTS I
      ON H.Symbol = I.Symbol
  JOIN
    PRICE P
      ON H.Symbol = P.Symbol
WHERE
        I.Product = 'stock'
    AND I.CategoryCode5 NOT IN ('X', '') ;