Cappytan has made some effort to try and work out whether the claims by this Marko and some of it that was published by Kim and Mike had any basis in truth or not.
Here’s why my gut says they’re fake. There is no known user interface, prior to 2016’s Microsoft Outlook redesign, that uses a person’s initials in a circle with a thin weight font to thread either chat or email messages. Prove me wrong. The implication here is that these screenshots were supposedly taken in 2008. I find that so far to be impossible.
I can easily find references to jQuery plugins for creating user-initials as avatars in a circle from earlier, at least 2014. Even Google were using them in 2015. So they were mainstream by then and straight away "There is no known user interface, prior to 2016’s Microsoft Outlook redesign" is invalidated.
If there were developer libraries for anyone to add them, then by that point it can be deemed to be a commonly used thing. So usage and actual invention could easily be from much earlier.
The challenge is finding the specific example, and despite what some people think, the internet isn't a great place at keeping history. Even things I know and remember, major products, are sometimes difficult to locate information about. People lose data over time, servers go offline, the past is often forgotten pretty quickly online.
So I doubt you can prove them fake and this claim doesn't do it as it relies on "lack of evidence" as evidence, which is weak, I think the only way they could have proven things fake would be if they'd used a screen resolution, font or anti-aliasing that wasn't definitively invented until some time later. But they don't AFAICT and would someone going to the trouble to re-create those old things in such detail then randomly decide to throw in some "modern avatars"? Or is the reality that we don't really know about the avatars?
Lack of evidence of something only proves a lack of evidence has been found, nothing more - that lack of evidence doesn't become evidence of anything in itself one way or the other. Technical analysis might have proven them to be faked, but if it doesn't it simply means it failed to disprove them as being credible.