I think there is a documentation "bug".
In 3.7 runtime for WebGL, "MixPose" is not defined. Looking through the runtime source, I think it is now "MixBlend" that is defined and exported. At least, I was able to call Timeline.apply() passing spine.MixBlend values. The docs here ( MixPose ) still say "MixPose".
No big deal for me. Just trying to help out.