Making globals readonly without fenv?

[FaultyPine]

By itself this would mean that code that runs in Luau can’t use globals at all, since assigning globals would fail. While this is feasible, in Roblox we solve this by creating a new global table for each script, that uses __index to point to the builtin global table. This safely sandboxes the builtin globals while still allowing writing globals from each script. This also means that short of exposing special shared globals from the host, all scripts are isolated from each other.

^ from luau.org
How does Roblox do this without calling lua_setfenv? Wouldn't you need to call that in order to replace the global table for a given script? If you do, doesn't that deoptimize the environment and is there a way to do this without that side effect? If not, how do you isolate scripts without manipulating the environment with fenv funcs?

Also, from some tests in a custom engine, I've found the follow lines of code to produce the following output, which is unexpected.

function SomeFunc()
    math.mod = function(a,b) return "HERE" end
    local test = math.mod(10,2)
    -- hit a breakpoint here
end

Printing out test and breaking in a debugger and looking at the value of "test"... I saw it contained "HERE". Luau makes those builtin library tables read-only, so i'm confused how this is possible. I do a similar thing to those last few sentences from the link above, where I override each script's global table and it's __index to point to the global table, so maybe that has something to do with it?

Does it have to do with https://luau.org/sandbox
And the features this refers to?

adding features to the VM that make it possible to implement sandboxing and isolation

TLDR: Looking for some clarification on what globals are/aren't read-only in Luau by default, and how to best prevent globals from being accidentally modified.