Since you are serious, when you have a septic tank in a derelict house you are dealing with, get the septic pumped. Getting a septic tank pumped is not that expensive, as long a the cleanout can be found fairly readily. The experienced septic pumpers know where to look. Getting a septic tank pumped earlier than needed is not expensive. A tank should be pumped well before any backups occur, but if you do start getting backups, act with urgency. I would guess an inspection would involve pumping first.
If you had no visible symptoms but don't know how long it has been since it was pumped, you would get it pumped.
My suspicion is that at some point squatters squatted there, but the water was not working. They pooped anyway, until the mound got high. The mound got shorter as it sat, decomposed, and dried out.
If you can get the water working, you would probably want to put in a new toilet and replace the fouled toilet, unless your time and effort does not have much value.
Whether you need to get the drain lines rodded, that is to be determined. But by my theory, there is a good chance no. I suspect they did not flush, and the crap did not make it to the drain lines.