That's not true. In most javascript implementations, integers will be represented by real 32 bit integers.
The problem with a naive average like this is that if you're averaging a bunch of big numbers there is a very high chance of overflow when you're summing, so even if all the numbers fit in 32 or 53 bits your calculation will be off.
If you're not averaging thousands of large numbers, why are you even using a package instead of nums.reduce((a,b)=>a+b) / nums.length anyway?