Biblical prophets could be chosen before birth, but John's wording in the context of the Prologue concerns the prior existence of the Word.
Jeremiah 1:5 offers a useful contrast between divine foreknowledge of a prophet and John's stronger language of prior existence.
Read after John 1:1-14, the Baptist's statement confirms that Jesus' existence did not begin with his human birth.
This is strong evidence for personal pre-existence, though this saying alone need not carry the stronger claim of eternal existence.