Again, I'm not sure how you can consider a company with a market cap of over 40 billion, and making 1.7 billion in revenue, some sort of "underdog competing against giants".
In their space, it's more like giants bumping each other around.
That's basically the definition of an underdog if you have a fraction of the number of resources (employees).
Which is not about the resources at all but about viewpoint on success.
So i'm going to say "no, it's not basically the definition of an underdog".
Even in slang, you aren't an underdog if you could have whatever you need, but choose not to. You might be one if you need something, don't have the resources to get it, and have to make do anyway.
That is not the case for Roku - they could hire as many people as they like.