Exactly this. You also need to ensure that the underpriced bet actually wins or, if your strategy allows for it, places in the top 3. The odds on a place are generally too low to make this worth it though.
The odds reflect the horse's supposed chance of winning or placing (or whatever else it takes to get a payout), not its expected position -- so this isn't a case where you can be aware that the horse is underrated, but have no way to make a +EV bet on that knowledge.
The first order of business in these things is certainly to spot the overlay/value pick/+EV situation. The second order of business is to manage bankroll/vol to be able to realize the EV over time -- but that is generally speaking much easier and more mechanical than the core insight of (repeatably) finding the EV...