Part of this debate is arithmetic, true by construction. Part is genuinely contested. Separating the two makes the argument far easier to follow.
Market Breadth: When an Index Is Not the Market
An index can rise while most of its constituents fall. That is not a data error but a direct consequence of capitalisation weighting.
What Happens When an Index Rebalances
A stock is added to an index and rises before the change takes effect. No analyst upgraded it. Several trillion dollars of tracking capital simply became obliged to own it.