I think the author means that vega and gamma will have opposite signs with the passage of time. being ATM, gamma of the short-leg goes up faster than the long-leg; short-leg vega goes down faster as well. that's why short 1m/long 3m will have more positive vega and more negative gamma as time goes by.
short gamma is usually profitable as the implied vol is always high than what the market has realized. being delta hedged, the option will gain more in theta decay than it will lose in gamma. if the market realized more volatility than implied, however, the strategy will lose money.
short short-dated vega and long long-dated vega (calendar) is a bit tricky. had a discussion with another trader several months ago. his opinion is that short-dated options suffer from selling pressure when approaching expiry (unwinding, rolling, etc), so the short-dated vol will go down more than the long-dated one. and this calendar spread is actually a steepener, you will make money from the increase in the slope.
so better not to sum up vega across different expiries, if you think the vol term structure will not shift in a parallel fashion. |
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