I don't think that would be the reason for that.
Especially since Google could've just designed it in a way so that either the earpiece speaker also doubles as a loudspeaker so that it either outputs higher-frequency sound while the bottom one outputs lower-frequency sound (like the HTC 10), acts as a separate loudspeaker for stereo audio (like the iPhone 7) or both depending on the device's orientation (like the Huawei Mate 9).
We might see either dual front-facing speakers or any one of the setups above in the 2017 Pixel, but the current setup seems to be more of an afterthought.
It's also worth noting that people who wear VR headsets like the HTC Vive, Google Daydream View and the Samsung Gear VR would also likely be wearing headphones.