Conversion to DNG doesn't involve any application of a tone curve. The tone curve gets applied when rendering a DNG to an output (e.g., for display or print purposes).
There is a very significant difference between "converting" and "rendering." As an example, consider opening a PSD file in Photoshop and re-saving it as a TIFF. All you've done is a conversion (of file format). There is no change in rendering. Now, consider taking that same image, and applying a tone curve to it (e.g., using Curves panel). In that case, you're changing the rendering.
Converting a raw file (like CR2, NEF, etc.) to DNG isn't rendering. It's just converting to another file format / container for the purposes of holding the image & associated metadata. The image data is still unrendered (scene-referred) e.g., no white balance, no color profile, no tone curve, etc. The rendering takes place when you open that DNG into a raw converter software and then save it out as something else (e.g., JPEG, TIFF), in a so-called output-referred format.