I don't think that's what the Godoc says. The IP type is defined as []byte

  type IP []byte

That is not the same as [16]byte.

  // Note that in this documentation, referring to an
  // IP address as an IPv4 address or an IPv6 address
  // is a semantic property of the address, not just the
  // length of the byte slice: a 16-byte slice can still
  // be an IPv4 address.

What I believe that comment is saying is that something that is [16]byte can still be an IPv4 address, but that doesn't mean that all IPv4 addresses are stored as [16]byte. At least that would be my interpretation based on the comment above it:

  // An IP is a single IP address, a slice of bytes.
  // Functions in this package accept either 4-byte (IPv4)
  // or 16-byte (IPv6) slices as input.