Yes that's why I wouldn't tax them extra, however you seem to be advocating for a tax on anything with sugar. So why the change? You say yourself they've still sugar in it so why not tax them according to your logic?
To put another way, if the presence of sugar isn't enough to justify the tax on its own, as long as the form is somrwhat different, then why not accept the difference between added sugar and no added sugar? That's just as meaningful a difference.