I know this agent has been an Allstate agent for a long time but I guessing Allstate is somehow allowing them to use another carrier. We are not in a rural area.
Allstate agents are able to use an internal MGA to access outside carriers for classes of business they can't write. So you could see American Modern for vacant homes, Hagerty for classic cars or even Hartford/Travelers/Liberty/MSA for business risk they don't write.
Most agents only use this to retain and/or help existing clients because it's an arduous process to work w/ their internal MGA. I'm sure if you lookup the agent's appointments on the DOI website in your state, he's not appointed with other carriers.