Is it true that marriage is not obligatory and individuals who choose not to marry but remain chaste and focus on their careers will not be considered sinful?
Yes
Marriage is recommended in the sunnah, and in a good situation, can provide stability, security, support, spiritual blessings, and happiness, and women in particular (even today) tend to enjoy a higher standard of living and more security when married. Of course, sometimes it doesn't work out well. Anyway it is optional, and some important sacred figures like Hazrat Maryam (a) were not married (in the view of Muslims).
This is a Shii view, and some Sunnis debate whether marriage is obligatory or not (although a good number of Sunnis would say it is not).