If he was on the clock while there, he could be accused of giving a sermon with taxpayers resources. Even if so, it was an innocent mistake. No one was hurt by it.
Absurd accusation, which would be Marxist in nature. They used the same gobbledegook to get prayer banned in schools.
But, as he noted, he's the Sheriff 24/7, whether in uniform or not - if a call comes in at 2 AM, he responds. The only potential harm being done is anything but an innocent mistake; it's a concerted effort by the commie faction to squash religious activities as much as possible.