This form of acquired vWD may be more prevalent than is presently thought.
presently (adv.) = at this time or period; now: He is presently staying with us.
An original meaning of presently was “at the present time; currently.” That sense is said to have disappeared from the literary language in the 17th century, but it has survived in popular usage and is widely found nowadays in literate speech and writing. Still, there is a lingering prejudice against this use. The sentence General Walters is … presently the United States Ambassador to the United Nations was acceptable to only 48 percent of the Usage Panel in the 1999 survey.
In your case in question "thought" is part of the understood passive construction
It is thought that these ares were reached in 1800 (passive)
People think that these areas were reached in 1800.