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.
Source? That would be surprising!