If this is for fire stuff, then I would assume that someone had to certify it, no? In that case it really doesn't matter whether they use a PLC there or an Arduino Mega and a custom relay board.
The Mega is obviously reading some sensors too, doing stuff like that (especially if math is required) on a PLC can be a royal pain in the butt if the sensors aren't the common industrial stuff (like thermocouples, current loops or something resembling a switch).
Another thing is cost - a PLC with this much I/O is going to cost around 1000 euro, depending on manufacturer, model and especially if you need fancier I/O then just digital on/off (whether relay or transistors switched). Then you may need development tools (depends on vendor - some like ABB have free tools, others don't).
Compared to that Arduino Mega is some 20 bucks (they even used a genuine one), the relay board is likely custom, but certainly less than about 100-200 euro. If it does the job and satisfies the requirements, solution like this could make a lot of sense. I wonder why they didn't integrate the ATMega MCU directly on the main board instead of using the Arduino module, but I guess they had some reasons ...