MR_SLINValve doesn't care about weapon balancing right now. Who cares if a weapon is overpowered in 6v6 if the final competitive format is 7v7? 4v4? 5v5? What if a weapon is overpowered with class limit 2 but not with class limit 3?
Whether an unlock is "overpowered" is hardly the point, you're missing the subtlety of the argument. Many of these weapons have been around for years and are aimed purely at casual play and have literally no place in a competitive environment yet their effects are so strong that they have to be included. The situation has been tolerable because whitelist is a thing, but if Valve are committing to MM then that's no longer the case.
Purely fun mechanics or mechanics made to make up for the inherent randomness of pub play can no longer be strong enough to have an impact competitively and they simply have to do something about it, once again the parachute is fucking ridiculous in this regard, the vitasaw is another example.
They don't need to consider "balance" to do this, they have to commit to a competitive mindset - it is a completely different matter. They can include weird shit that is basically ineffective and players who just like to pub with odd loadouts can still do so. From a game design perspective that's both fine and necessary.
"Balance" isn't the key, it's about putting the skill of competitive play first. You can't have a no-whitelist MM without making those changes and have it taken seriously, it doesn't matter how many players you have or what ruleset, in fact changing them is probably a prerequisite for any format to work. Some items are simply too powerful for the low skill anti-competitive mechanic they introduce and you don't need to argue about balance to change them.