kotlin companion object constructor

I'm studying Kotlin with design patterns.

So I write a singleton pattern.

Below code is test code. first show that instance is only one. second show that there is no public constructor.

import org.junit.jupiter.api.Assertions.assertEquals
import org.junit.jupiter.api.Assertions.assertSame
import org.junit.jupiter.api.Test

class TestSimpleSingleton {
    @Test
    fun testCreateSingleton() {
        val s = Singleton.Instance()
        val s2 = Singleton.Instance()
        assertSame(s, s2)
    }

    @Test
    fun testNoPublicConstructors() {
        val singleton = Class.forName("Singleton")
        val constructors = singleton.constructors
        assertEquals(0, constructors.size) // public constructor = 0
    }
}

if i use object to implement singleton the test is work well.(val s = Singleton)

object Singleton

but i use a companion object it doesn't work.

class Singleton private constructor(){
  companion object {
    private var theInstance: Singleton? = null
    fun Instance(): Singleton {
      if (theInstance == null)
        theInstance = Singleton()
      return theInstance as Singleton
    }
  }
}

I write private constructor so I thought it also didn't have any public constructor. but the second test shows that it has one public constructor.

Why did it happen like this?