Which tweet are you talking about exactly?
As for her dating Chad before his divorce was final...so? Not to defend him or anything, because lord knows I dislike him and hooking up with a teenage extra, who primarily had to work with Sophia was a shady thing to do imo, especially so soon after the split. But him still being legally married isn't that big of deal imo. Sophia made it clear when they announced the split that she was done with him and wanted to move forward and forget the marriage ever even happened.
So they were both free to see other people. Now he moved on really fast, but Sophia began a relationship with Jon in 2006, while still legally married too, since the divorce wasn't official until december of that year or something. So both of them were in other relationships while still technically married, even though one moved on to another relationship a lot faster, and even went as far as to get engaged, which I found ridiculous, but hey, it's heir life. Clearly didn't work out in the end anyways.
As for us never finding out the truth and what really caused the split and if cheating was involved...well that could get out at any point. Never say never.
Seems like a lot of people have already made their mind up though and are throwing the "once a cheater always a cheater" line around, me included. Might be the truth, might not be, but it would not be at all surprising. I'd actually be surprised if that wasn't the case.