It’s not a mistake I do personally.
However, your reviewer might want you to do this for better organization and namespacing. You are importing the globals and not declaring them. In this case, you cannot control the name of the variable and now the variable $post or $paged is referencing the global variable post and paged accordingly.
Whereas if you used a function, you can prefix the function making it unique to your theme.
Note below, this is what the original poster is doing:
global $post, $paged;
What this does is imports the global $post into the variable $post at the file level. This variable is now available to the whole file and any other files included.
So a third party plugin, unaware of your usage of a variable $post or $paged within file scope could do :
$post = “bubba”;
You have now lost the original value of $post as the variable is not unique.
Your code tries to access $post and ends up with nothing but “bubba”. Bubba, who the hell is bubba?
Conflict 
So I think, but not your reviewer so just saying what makes sense to me and why I wouldn’t import globals at the file level 