[ALEX]

Considering a combination of an item and provenance, not history alone, is a proper approach. This particular item calls into question the story as a whole. Firstly, it is an Indian Mughal style, but was it made in India? Could it be made in Japan, which could explain anomalies from traditional Indian Mughal craftsmanship? Secondly, and as noted by others, the integrity and workmanship of the dagger and the case is questionable for supposedly high status item. The theory of put-together item, and the provenance, is also quite plausible, considering all the above and the Italian auction houses reputation... there's a meat cleaver being pitched as rare important executioners sword by one of them... also hope I made no offence sharing this.
The auction's description about a tassel being "definitely added on shogun order" falls right in with the rest of the story... the provenance based on a small porcelain ball and carved characters on the wooden box.
This well could be as claimed, and seems like those who bid on it thought as such. the opinion expressed is strictly personal.
Also, below is an image with the blade shown.