I played Civ 5, then Civ 6, on my 2013 Macbook Pro for the entire life of the machine (about 4 years), and currently play Civ 6 on my 2017 Macbook Pro. It will get alarmingly hot, pretty much every time. Like, so hot if you rest it on your lap you need a book to keep it from burning your legs.
But after researching it in 2013, and being told it was normal for the Mac to disperse heat in this way, I stopped worrying about it. And 4 years later, I don't believe it affected the life of the machine in any noticeable way (except making the fans work harder, and thus probably leading to more dust inside the computer). (So make sure to keep the air intake ports on the side as clear of dust as possible--it really does make a difference.) But that was just my experience, everything I read online does say heat can affect the life of your CPU, so really take it with a grain of salt.
A difference I have noticed w/ my 2017 machine, is that the CPU throttling when it gets too hot is actually way more noticeable. So keep that in mind if you're finding the game performance fluctuating.
I don't think you have anything to worry about. Looking at the way that model of MacBook is built that looks like the way the heat-sink has been designed to dissipate the heat. Macs are very clever and will auto-shutdown if they do get too hot.
Civ 6 was the game that finely convened me to upgrade from my 2014 base MacBook Air. My Air would get pretty hot, but it'd also take like 5 minutes for the computer to take its turn later in the game. So I just got a base MacBook Pro 2017, and while the speed is there it still gets pretty warm, but I can still play while its laying on my lap. Part of the Macbooks thermal dispersion is through the chassis, this is normal.