That's his opinion, but without a free and comprehensive investigation, no one can say for certain whether Trump lost legitimately or illegitimately.
What we do KNOW - not speculate, but KNOW - is that there were widespread shenanigans in many "swing" states. Did those shenanigans change the "legitimate" outcome? We can't know, because a proper, comprehensive investigation was never done. Based on the audits that were done, it certainly appears one or more states' outcomes were not legitimate.
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