the sensitivities you are referring to are local derivatives. SimBiology integrates a system of differential equations, using complex step differentiation, to compute the derivatives. More details and references for the methods are documented on this site in section Local Sensitivity Analysis (LSA).
It sounds like you are more interested in Global Sensitivity Analysis; specifically Elementary Effects, since you mention perturbing sensitivity inputs by a certain fraction of the size of the parameter domain? SimBiology currently (Matlab version R2020b) has features to compute Sobol indices and a multi-parametric global sensitivity analysis. Elementary Effects are on the list of new functionalities to be added in a future release of SimBiology.