We have no way to accurately determine how fast the thing was moving because it isn't even a real thing we know the speed of IRL. Its only feat is outspeeding Reimu and Marisa, which means it also has infinite speed. It's not just a regular animal, y'know.
Sakuya being stated to be FTL can mean literally anything; sure, by itself, the statement only implies low FTL speeds, but it could just as easily mean MFTL+ or infinite since, y'know, those are both above the speed of light. And we have several explicit feats showing those speeds.
Even taking everything you're saying at face value, this would be a massive outlier for a series that quite regularly displays FTL speeds and much higher. We wouldn't really use it regardless of its accuracy if its too far out of line with what's consistently shown.