The rationale for the Statute of Limitations was explained in one of the Case Evidence episodes IIRC. Basically, the State of Georgia requires that Law Enforcement know of both the act and the person for the clock to start ticking. IOW, even if Law Enforcement knew a crime had occurred, then the Statute's clock wouldn't start ticking until they had a suspect - and only for that person, no one else. I'm sure that the DA's argument is that, when the initial search happened, the only name that they had as a suspect was Ryan's. Hence, Bo's clock didn't start ticking until more recently when his name came to Law Enforcement's attention. I'm sure that Bo's Defense Team could appeal. It was on Bo's family land and Ryan was his roommate, so it'd seem that there could be some wiggle room there for an argument. No idea of the level of proof the defense will need to show to overturn the conviction though.
But didn't Phil Holloway say that all they have to know of is that an act occured???
Bo was convicted for his lies, which were in 2016, and in a different county (Wilcox) than where the body was burned (Ben HIll). Thus the crime was what occurred in 2016 and the SOL begins to run then. Charges related to the actual burning of the body are a different story.