Why should you use newproxy(true) over setmetatable({}, mt)?

So I was curious and started to inspect code from Freya which is a framework for Roblox. I was reading the code for Arbiters and saw this code:

--code
Controller.new = Hybrid(function(f)
  local newobs = newproxy(true)
  local omt = getmetatable(newobs)
  for k, v in pairs(ObserverMt) do
    omt[k] = v
  end
  ObserverCallbacks[newobs] = f
  ObserverEmitters[newobs] = Event.new()
  ObserverRules[newobs] = { }
end)
--more code

The part that I'm curious about is

local omt = getmetatable(newobs)
for k, v in pairs(ObserverMt) do
  omt[k] = v
end

I searched up on the wiki about newproxy and all it does is create a blank userdata.

Why not just do local omt = setmetatable({}, ObserverMt)?

What does newproxy(true) really do?

Does it have an effect on metatables? If so, what is it?

Userdata has stronger protections
No, don't touch that

Even though Freya has this whole fundamental concept of open data, a lot of the objects rely on being locked down. Why, you ask? Two reasons:

• If it comes from Freya it should behave predictably
• It reduces the surface for attacks which target Freya as a vulnerability.

Overall, the newproxy method can have a slightly larger memory footprint, but proper structure optimizations can nullify this and even present advantages over tables. Userdata also can't have any metamethod invocations bypassed through use of the rawset and rawget functions.

Back to the real question

The metatable must be set using getmetatable because setmetatable only works on tables. It's a small price to pay, given Roblox's modification of newproxy to remove userdata templating.