Is Sheriff D'Agostini Interposing or Following the Law?

I say he is just following the law; at least when it comes to his relationship with the Forest Service.
Institute on the Constitution and League of the South board member Michael Peroutka’s commentary today claims El Dorado County Sheriff John D’Agostini is following the principle of interposition by suspending the powers of the United States Forest Service in his county. Peroutka wrote:

Fed up with the repeated harassment his constituents were experiencing in the national forest within his county, Sheriff D’Agostini informed the feds patrolling the forest that their powers there were suspended.

Peroutka holds the view that local officials may take jurisdiction over federal agents when those federal agents seek to enforce a law the local authorities deem unconstitutional. At least that is what he told the League of the South.
According to this report, D’Agostini has removed enforced power from USFS personnel because they were not conducting themselves in line with his expectations.

It may sound like he is overruling the feds, but he is not because, as the professor on the clip said, he actually has authority under California law to do so. CA code 830.8(a) provides authority for federal agents in CA under certain circumstances. However, certain federal personnel are exempt and only have authority if given by the local sheriff. Note those who require local consent:

This subdivision does not apply to federal officers of the Bureau of Land Management or the United States Forest Service. These officers have no authority to enforce California statutes without the written consent of the sheriff or the chief of police in whose jurisdiction they are assigned.

According to CA law, the local sheriff can deputize USFS personnel but can also revoke that authority. On this talk show, D’Agostini clarifies that he is acting consistently with CA state law. He also notes on the talk show (at 16:53) that the USFS has federal authority to enforce forest service travel rules. While D’Agostini may or may not like those rules, he is clear that the forest service will continue to enforce them.
Thus, speaking of the suspension of federal powers is not accurate. Prior to D’Agostini’s actions, the USFS only had powers granted by the sheriff pursuant to state law. Any actual federal powers remain as before.