I should say it’s “mostly” a myth, there are some fleeting competitive advantages to first mover but a lot of them don’t apply well to tech companies and there isn’t strong historical evidence supporting it.
Why? Being a first mover only counts for something if it can yield exclusivity that is durable.. you should know this being a VC and all. Real options - hello?
If you want to benefit massively off being a first mover, you better do the work in figuring it out how you are going to acquire exclusivity that lasts long enough that keeps most firms out.