Right they can't speak of themselves, because there are people who have no sin, for example those who are cleansed from all unrighteousness. I mean the same John tells you that in the very next verse doesn't he? You can't make a doctrine out of only one verse.
1 John 1:9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
So, assuming you believe Scripture, you would believe He cleanses us from all unrighteousness, not just some, you would know that these people who are cleansed have no sin in them, since they were cleansed from all unrighteousness, not just some.
And none of their sins shall be mentioned unto them, that is Ezekiel 33:16. Which means they are both righteous and without sin. John was merely speaking to the ungodly, before they confess, not to the righteous after they were cleansed from all sin. In context John says that if the ungodly say they have no sin [to cleanse] they deceive themselves, for all have sinned [in the past].
Because the same John will also tell you:
1 John 3:6 Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him.
7 Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous.
So, we see that according to John, sin is only found in those who have not seen Him, neither known Him, not in those who abide in Him. And also John testifies of the existence of the righteous in verse 7.
And this is from John only, we find both the righteous and the sinless, they are the same, cleansed from all unrighteousness. Because you will find them also in the Psalm 119:3 which you haven't commented on, the Words of Jesus who testify of the existence of the righteous which is more than enough to prove they exist, saying they don't is to speak against Christ.
And a reminder that God will not justify the ungodly because He cannot lie. He will justify the righteous, because the righteous is righteous indeed.
