I'm not sure what makes this a specifically startup marketing plan. This a very traditional marketing plan template. In fact, I would argue that a startup in the pre product-market fit stage of development would be advised against sticking to a plan like this, since things like positioning, target audience and strategies could change month-to-month and even day-to-day.

But if you were following this traditional marketing plan, I still think there's a very important section missing and that's measurement and testing. This is one of the more important parts of your marketing plan. You should devise your tests with thought and purpose behind them and not as an afterthought.