Robertson, who at the time of the Jan. 6 attack held a position of public trust as a police officer, absolutely deserves this sentence. His participation in the attempted coup at the Capitol - for which he demonstrated no remorse - showed a lack of regard for public safety, anti-American sentiment, and threatened democracy.
It's absurd to call Jan. 6 an "attempted coup" by unarmed citizens who largely committed no acts of violence. The only ones who truly represent anti-American sentiment are those who are targeting patriotic citizens and labeling them as "domestic terrorists".
There's a 50% chance that 19 or more people will be charged by the US DOJ with seditious conspiracy in connection with the Jan. 6 Capital riots, according to the Metaculus prediction community.