Not true. Inclusion in a widely-invested index almost certainly means an increase in the stock's price, all things being equal. This will be most visible up front as index fund managers buy in (because they have to). But there will also be a much larger base of investors that hold this permanently insofar as they hold S&P 500 index funds (or ETFs, CTFs, etc.). This could directly provide the company more "currency" to make purchases of other firms (by paying in stock), as well as give it an advantage in attracting and maintaining talent that are partially compensated in stock (assuming they award stock/options).
It could also result in more stock analyst coverage which would bring valuable publicity to itself and its shares, and likely also more "mind share" for Linux for the average investor that follows this sector/industry.