After the for loop iterates through the length of the level, just add the node that you get after popleft. Extra logic, does not make much sense.

from collections import deque

#class TreeNode:
#  def __init__(self, val):
#    self.val = val
#    self.left, self.right = None, None

class Solution:
  def traverse(self, root):
    result = [] # List[int]
    if root is None:
      return result

    # TODO: Write your code here
    q = deque()
    q.append(root)
    while q:
      level_size = len(q)
      for _ in range(level_size):
        node = q.popleft()
        
        if node.left:
          q.append(node.left)
        if node.right:
          q.append(node.right)
        
      result.append(node.val)

    return result