Conspiracy theory ahead: I think the whole mutiny was planned to involve atomic weapons, which would have given Wagner a waaaaay bigger bargaining chip.

Between the southern headquarters and Moscow is a nuclear storage site. To arm the warheads one needs a code which is only known to three people: The president, the minister of defence and the chief of the armed forces. It just so happened to be that the latter two where expected at the southern headquarters at the day of the mutiny. Except they were nowhere to be found, possibly tipped off by someone.

Now Wagner had a problem: They failed to capture the codes but couldn't simply back down without some form of deal. So the convoy to Moscow went ahead anyway to put pressure on the government.

They initially succeeded as in they secured an exit strategy via presidential pardon but lacking the nukes, there is no chance of any meaningful retaliation by Wagner. This means the pardon was easily "rescinded".