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][1] 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 $\ce{Br}$, which 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.

-----------

$$d[Br]/dt=k1.[NO2Br] - k2.[NO2Br][Br] - k3 [Br]^2$$

For the dynamic equilibrium of the steady concentration of $\ce{Br}$:

$$\begin{align}
0&=-k1.[NO2Br] + k2.[NO2Br][Br] + k3 [Br]^2 \\
[Br]&=[ -k2.[NO2Br]+sqrt((k2.[NO2Br])^2+4.k3.k1.[NO2Br])]/(2.k3) \\
d[NO2Br]/dt&=-k1.[NO2Br] - k2.[NO2Br][Br]\\
\end{align}$$


  [1]: https://en.wikipedia.org/wiki/Rate_equation