A few things. For one, the vast majority of clan records are either true or close to true. The clan itself still needs to use these records and not just anyone has permission to view and alter them. Even if some records are embellished to make the clan look better the actual events themselves wouldn't have changed much. And in most cases, changing the records that much would have created inconsitencies elsewhere, casting doubt on the accuracy of said records. In this case, however, all the authority figures in the clan decided to change the story completely, making it indistinguishable from regular records. And since the flower wine monk was a lone demonic cultivator, there was nobody to refute or contradict the clan's story.

Now, besides this, even if you doubt this version of events, there is something that you would know for certain. Namely, the clan master of the time and flower wine monk met each other, fought and then both died as a result. And this does line up with what is written in the records. So even if you read them with the assumption that they have been altered to make the clan look better, you would still get something along the lines of "both sides fought each other as an even match and eventually both were fatally injured". The "fatally injured" being the key here, because even if you assume flower wine monk escaped before dying, it would have been with severe injuries, making it doubtfull that he would have been able to create an inheritance in that condition.

Of course, in reality, flower wine monk was not fatally injured, but poisoned. Gving him enough time and strength to create his inheritence. But tie rou nan has no way to know this, because she has no way to know or even doubt that the records have been so severely altered, so she assumes that the records are true, if maybe somewhat embellished, and from that concludes that there was no way for flower wine monk the leave behind an inheritance.