The presence of the footnote indicates a revised interpretation by someone involved with that version. While I cannot speak with authority as to their motive, it is self evident that someone did indeed 'edit God's Word', and this was acceptable enough to the recipients of this particular version that the footnote still stands. I do acknowledge that my inference that it was done to alleviate confusion regarding the literalness of a biblical 'day' is merely my own opinion.
It is not a simple task to translate ancient Hebrew (for example) to modern English. Translators can go either the literal word-for-word route (although there are sometimes multiple meanings for Hebrew words), a phrase-for-phrase route, or a meaning-for-meaning route. The translators of the ESV saw that there were multiple possible ways of correctly translating that sentence, and offered the alternate translation as a footnote, not as a way of adding their capricious editorial comments.
It puzzles me that you'd be so concerned about the accuracy of biblical translation and interpretation, though, considering you see much of Scripture as parable and myth.