This feature wasn't mentioned in the product announcement event.
There's likely a simple explanation for that: Apple Watch doesn't really gain substantial benefit from 5GHz WiFi connectivity.
5GHz can provide more bandwidth but does so at shorter range than 2.4GHz connections. However it's not like the Watch is streaming 4K@60Hz video via WiFi.
Having access to the 5GHz spectrum might reduce some interference issues compared to the overcrowded 2.4GHz spectrum (which often receives additional problems from microwave ovens, fluorescent lighting and devices that typically need longer range like baby video monitors) but it is unlikely that 5GHz is a game changer for a relatively low bandwidth and low-power device like the Apple Watch, especially the LTE models.