Perhaps Brown is expecting its engineering majors to enter already knowing the content of Math 0090 (calculus 1) through AP or some such. Probably they just do not want to deal with somewhat unusual situations (i.e. ready for calculus 2 but does not have AP or transfer credit for calculus 1), and the math + natural science volume (32 credits equivalent) is enough for ABET requirements without including Math 0090, so they just leave it out of the requirements list. But a Brown student who needs to start in calculus 1 would need to use up one of the free elective slots for it, reducing the free electives from 5 courses (20 credits) to 4 courses (16 credits).
For AP calculus scores, Brown says that 4 on AB or 3 on BC counts for Math 0090 (calculus 1), and a 4 on BC counts for Math 0090 and 0100 (calculus 2): Advanced Placement Exams | The College | Brown University