The onset of Shabbat certainly may start at shekia at the biblical level (d'orayta). However, there is a biblical uncertainty (safek d'orayta) whether this is indeed the case or whether the onset of Shabbat is at tzeit hakochavim (the emergence of the stars).
Additionally, it is disputed when exactly shekiat hachama occurs. Many medieval scholars assumed that the phenomenon was not really referring to sunset (e.g. Rabbeinu Tam) and there was a custom to assume that it was later. There was also an opinion that it was earlier than sunset. Nonetheless, later authorities, basing themselves partly on the difficulty of such a position when followed in northern European cities such as Wilnius and Prague, assumed that shekiat hachama does indeed refer to literal sunset. This also turned out to be the position of the Geonim, and seems to be the view of Maimonides as well.
Nonetheless, the Talmud is quite clear that shekia is potentially the d'orayta cutoff, and if a person unintentionally violated the Shabbat after shekia but before tzeit both at the beginning and end of Shabbat (the latter earlier in the period than the former) he would definitely be obligated in the chatat offering.
Still, because of the uncertainty, a well as the position of Rabbeinu Tam, there are those who have certain leniencies in times of great need.