You guys crack me up.
The situation is not as black and white as some of you may think. ACE is using Unity true, but that doesn't mean they're handcuffed by the limitations of the engine. We don't know what parts of the engine they're using and how they're using it. A game engine only becomes a limitation when you are stuck to the confines of what its capable of doing. For example, maybe you needed a specific algorithm that solves a problem in a precise way. If you're not able augment the current engine with your own unique code, then you're stuck in a box until Unity adds that specific feature to their engine.
Let's not forget that the current environment has debugging enabled on both client and server-side. Debugging is extremely CPU intensive. With debugging off, you could see a sizable increase in performance depending on the scale of debugging...how much data they're capturing.
Also, let's consider server side performance. I'm not sure if they're using containers or VMs...but...despite what you may think, this is not a single server with a maximum capacity of 2500 players. Every single zone you load into is a server that lives in a larger cluster.
Speaking of server performance - to make it relatable to you guys, think of GPU performance. Let's say you spend $500 on a solid mid-tier card and you're feeling pretty great about it. You also have the latest and greatest 120HZ monitor, so you have to maintain - at minimum - 120 frames to take advantage of the monitor's capabilities. You load up a semi-intensive graphical game, and notice you're only able to maintain an average of 75-80FPS. You spend hours tweaking graphical settings, your system, hell you even decide to start overclocking. God damnit you spent a lot of money, you're going to squeeze out every damned frame your hardware is capable of. Maybe you push your average to 90. Well, that's great but it's still not good enough. You decide - screw it - for my purposes I guess 90FPS will have to do. I really don't want to drop $1000 or more on a high-tier card to get to that 120 mark.
This is the most likely scenario of what ACE is dealing with. At this given moment, they can't justify spending the extra money that would allow their server cluster to auto-scale beyond the limits they have set. So they decided to address the problem as you did with your GPU tweaks. It's a bit more complicated than google searching a GPU overclock template, but my guess is they made improvements. Those improvements may have made things more efficient, but ultimately the bottleneck still occurs. The point is, no matter how well constructed your code is, in some cases you're going to hit a hardware wall. To get past the hardware wall, you need to scale. To scale you need money. So, do they spend the money now - in an Alpha build - on the server infrastructure? Even if it was to prove a point, it's a waste of money. In the process of checking their code and performing tweaks, they may have been wanting to know - ultimately - did we screw up in our design or are we hitting the hardware wall on our test servers?
So yes, they may have tried to improve server performance, but just like you, they decided "screw it. For now, this will do." That doesn't mean the "unity engine is trash, this game is done." My advice is pretty simple. If you don't know how any of this works, you should be skeptical on what information you allow to accept as truth. Being negative for the sake of negativity has never and will never serve any meaningful purpose in any aspect of life. There was some mention earlier about Albion Online developers complaining about the Unity engine, as if it is the reason their game failed. If true, that's a pretty bold statement considering their initial network and server infrastructure was developed by a 5 year old. Oh yeah, it was a Unity Engine issue for sure.