Can I use delta as a proxy for probability ITM/OTM?

Yes. The reason why is because the more moneyness, the higher the option price's sensitivity to the underlying and thus the higher the Delta (remember Module 2 Lesson 2 timestamp 06:23).

Essentially, because of the way Delta increases the more ITM the option gets, it's a proxy for Probability ITM. There's a real mathematical reason behind this... Calculations for Probability ITM (and Probability OTM too, for that matter) are all based off comparing the DISTANCE between the strike price and the underlying price vs. the stock's historical/typical range of movement. In a nutshell, Probability ITM calculations are based roughly off of an option's degree of moneyness, and so is Delta, hence they can be used as proxies.

If you're going to use another brokerage instead of thinkorswim, Delta is an acceptable proxy. You'll just want to make sure to use the INVERSE... i.e a Delta of 0.90 means Probability ITM of 90% and Probability OTM of 10%. So if you're targeting say, 70% Probability OTM, you'd want to sell the 0.30 Delta strike.