Small bits of tru-oil rag won't combust. You need a really big bit soaked in tru-oil or linseed oil to get anything like hot enough for it to catch fire. And unless you've spilt a whole large bottle of the stuff and wiped it up with a whole T-shirt, then that isn't going to happen.
Tru-oil and linseed oil dries by oxidising (which creates heat), rather than evaporation of a solvent (which cools the finish down), but you need a lot of it, insulated by being in the middle of a bundle of rag for the temperature to rise much above room temperature. YouTube videos always show a whole bucketful of soaked rag, which is not a real-life scenario for the kit guitar builder.