“Exonerated” means there is proof of non-guilt and generally applies to a situation where there is no evidence of guilt (due to presumption of innocence). Declining to prosecute does not imply exoneration. Prosecution requires that evidence of guilt is provable, that the law in question is prosecutable with this set of evidence and this individual, and if the prosecutor deems the good of indicting this individual is worth the cost of prosecuting this crime.
For instance, if DNA evidence is found at a murder scene which clearly came from the killer but does not match the suspect, the suspect is deemed exonerated. Doesn’t mean the suspect is necessarily innocent (maybe two people were involved in the crime?) but there is no remaining solid evidence pointing to that person being guilty.
In the case of Trump, exoneration would have meant that a sound and reasonable explanation could be given for all cases of obstruction, supported by documentation at the time. It appears physically impossible for Trump to be exonerated of the act of obstruction as well as of intent to obstruct given his public statements at the time. The only way for Trump to be “exonerated” of obstruction charges is for Barr’s strained theory of proof of an underlying crime to hold sway, in which case “exoneration” of conspiracy (and all other crimes, which Mueller was not tasked with investigating) would imply exoneration of obstruction.
The bottom line, though, is simple. Mueller specifically said that Trump was not exonerated. Thus, Trump claiming Mueller exonerated him is severe gaslighting. Which will, as history has proven, work about 100% on Trump’s supporters who trust what their leader says over their lying eyes every single day.