Depends on how you define "being based on". Metal in so far based on OpenGL ES 3.0 as they took the latter as a blueprint to decide what the former should be able to do. As a result, the feature set of the first version of Metal (on iOS) was pretty much identical to OpenGL ES's. (And yes, this feature set was (and still is) limited when compared to Vulkan, DirectX or even full OpenGL.)
On the other hand, Metal is in fact not based on OpenGL ES 3.0 in the strictest meaning of the word, as they share no code base and are technically quite different.
(I just wrote the same over at the IMG forums – so if this sounds familiar to you…)
An API, and the feature set of an API are two very different things.
The original poster was claiming that the actual Metal API was based on GL ES (in terms of actual API design). Which it very much isn't in the grand scheme of things.
For instance, Vulkan has multiple feature sets. Including one at it's base level that's similar to GL ES in terms of features (it excludes tessellation, and various other things). But the actual API wasn't based on OpenGL. The same is true for Metal.