There is a slight flaw. She is only correct if all three points are on the same line.
If S is on segment RT, then segment RT can be broken up into RS and ST. Through the segment addition postulate, we can then say
RS+ST = RT
So put another way, Michaela's claim is only true if angle RST is 180 degrees.
However, if angle RST is some other angle, then saying RS+ST = RT is not correct. So Michaela needs to clarify that point S is on segment RT. Or she could say that all three points are on the same straight line.
A diagram is provided below.
Side note: in the figure on the right, RS+ST adds up to something larger than RT. This is due to the triangle inequality theorem. That theorem states that adding any two sides of a triangle adds to something larger than the third remaining side.