I don't see why new hardware would be needed for a very small increase in cell voltage. It might go up a few percent, but the hardware should already be able to handle cell voltages of 5V or more.
If you read the SA article, you'd understand that the 85kWh/90kWh batteries in the S/X when you consider them as a whole pack, are already at a voltage which is approximately at the limit of what a supercharger can provide. If you increase the cell-level voltage, thereby increasing the total pack voltage by the difference multiplied by the number of series groups, and don't change the configuration of the pack to have fewer series groups, then the supercharger won't be able to charge it fully.
Since its unlikely they're going to change the architecture of the pack layout from the ones they've already got, due to design limitations, it makes sense to use the 14 module 60/75 pack, but without the dummy cells, as a base for the 100 (based on higher voltage cells), instead of starting from the 16 module 85/90, which is already pushing the limits of what a supercharger can do. That way, the higher voltage is across fewer series groups, and can stay within the limits of the already deployed superchargers.
The alternative, of simply adding more cells in parallel would work, but comes at the cost of longer charge times, which are also undesirable. Its all just design tradeoffs, and I think it unlikely that slower charging or an inability to fully charge via supercharger are things that Elon would choose for the top battery, your analysis may well be different than mine. The alternative is retrofitting the superchargers to be capable of a higher voltage -- seems too early for a revamp like that.
The main part of my interest in that article is really in the evidence that suggests they're already shipping cells with a new technology, given the minimal difference in curb weight between 75kWh and older non-upgradable 60kWh models. If the gains were made by simply adding more cells of the older tech, then it should be accordingly heavier. If they were from chemistry, then it would be about the same weight since electrolytes will all have approximately similar mass, and if it has similar cost, would explain why they were able to reintroduce the 60kWh as a software limited thing without really demolishing the margin. (because the 60 would still have the same margin it always did, and its actually that the 75 ends up being good for margin to the tune of 9k.)