It is more probable like $$\begin{align} \ce{NO2Br &-> NO2 + Br} \\ \ce{NO2Br + Br &-> NO2 + Br2} \\ \ce{2 Br &-> Br2} \\ \end{align}$$
The last reaction is a minor one in case concentration of $\ce{Br}$ is low.
The reaction rate order can be concentration dependent and need not be the integer.
In fact, it is rather mathematical parameter, related to solution of differential equations for a complex reaction system.
If the 2nd reaction is fast enough, the overall reaction rate is given by the slow rate of generation of $\ce{Br}$, The fast reacts to form $\ce{Br2}$
If the 2nd reaction is slow enough, it's rate $$k_{\rm 2}\cdot [\ce{NO2Br}][\ce{Br}]$$ can be written as $$k_{\rm 2a}\cdot [\ce{NO2Br}]^2$$
The exact solution is to solve system of differential equations for the rates of the concentration changes.