I'm reading a paper that tries to estimate the following model: $$y = \alpha x+\beta z+\gamma xz+\epsilon$$ where x,z are dummy variables and y isn't. The estimated coefficient of $\alpha$ is positive (0.02), and the sum of $\alpha+\gamma$ is negative (-0.01,insignificant). The writer examines the marginal effect of x, conditional on z and finds a positive (0.01, with significant of 90%). My question is what is the difference in interpretation of marginal effect VS. $\alpha+\gamma$?
The paper is "Terrorism and Voting: The Effect of Rocket Threat on Voting in Israeli Elections" by Anna Getmansky and Thomas Zeitzoff (link: https://www.zeitzoff.com/uploads/2/2/4/1/22413724/zeitzoff_getmank_rockets_main.pdf). The marginal analysis is on page 11.