Leverages are risky to use with money and in a dull market, shorting or longing with leverages is very risky. It's not only "Never short a dull market" but it is true to say "Never long a dull market" if you are a trader, not an investor with plan for long term holding. Obviously Long and Short are used for leverage traders, not for investors.

In a dull market, excluding investors, with traders it is safest to close all open positions, exit the market and wait for big movement to join the market again.