Data type detection in Tableau
Yes, it sometimes does that. I think it depends on the data source. For RDBMS it is likely that it takes the data type from the database (so the column type is typically accurate). For text/csv, it probably does a scan of the top certain lines to guestimate the type. It is similar to the text import wizard of the SQL, etc. other similar tool.  (A Tableau user of 4+ years)
I am not sure if it is relevant to many of the Tableau users; many databases don't have that distinction either; I think it is more helpful for someone working in machine learning, etc.?