This isn't true. If it were, you would have an arbitrage. Specifically, for a calendar to be strictly better you would be saying there is a systematic mispricing in root time vol. At the end of the day, the calendar and the strangle trade the same basic view that the earnings vol (vol crush pnl) is > the actual earnings vol. The difference is the strangle will have a different convexity profile and the calendar will have risk to where the vol resets. Both are different varieties of chicken on the menu. Perhaps today you want a fried chicken and tomorrow you'll want a grilled chicken.