Largest Prime Factor

Euler3.fs

open System

let isPrime (n : int64) =
    let upperBound = n |> float |> sqrt |> int64
    seq {2L .. upperBound}
        |> Seq.forall (fun x -> n % x <> 0L)

let getAllFactors (n : int64) =
    let upperBound = n / 2L
    seq {2L .. upperBound}
        |> Seq.filter (fun x -> n % x = 0L)

let largestPrimeFactor (n : int64) =
    getAllFactors n
        |> Seq.filter isPrime
        |> Seq.max

I've seen solutions where the upper bound when finding the factors of a number is the square root of that number. But isn't 5 a factor of 10?