A neutral map would be very unlikely to result in a Pub gerrymander in IL. VRA decisions make it impossible to pack minorities to the extent required to get a Pub win with an intentional gerrymander barring a wave election. A neutral map would obviously not do as well as an intentional Pub gerrymander, so it would be less likely to give Pubs an advantage.
Would neutral maps result in frequent situations where Republicans win a majority in one or more chambers even while losing or possibly tying the popular vote for said races? This is sort of why I wanted to ask - an "indirect gerrymander" seemed sort of extreme, but given how densely packed voters appear to be in Illinois, the idea that Republicans would have a consistent advantage and thus more frequently win large amounts of seat with comparatively less effort seemed likely based on his assertions.
That sounds correct. If you look at presidential elections results, you notice that major cities concentrate large number of Democratic votes in a way that GOP leaning areas don't. To use your Illinois example, 2 million+ votes came from Cook County, which voted over 70% Democrat. Even in a 50/50 election, nowhere near that many GOP votes will come from 70%+ or even 60%+ GOP counties.
Since the GOP is so much less concentrated it tends to show up more in counties where they win 55% of the vote. This in turn suggests that Illinois itself is something of a Democratic pack. You can repeat this example across states with major metros. Of course that doesn't necessarily mean GOP majorities as most of these states aren't 50/50.
The most dramatic part of the current gerrymander is around Chicago. To fully utilize all of the Dem voting power in Chicago there are lots of fingers out from the city into the nearby suburbs. From 2000 to 2010 Chicago lost about 200 K in population equal to two house seats. The gerrymander kept the size of Chicago's delegation unchanged.
The flip side is that Dems are also concentrated in downstate cities like Rockford, Rock Island, Champaign, Springfield, and a lot of Metro East (across from St. Louis). The populations are small on the congressional scale but can dominate legislative districts. A neutral map that respects political boundaries would protect those areas, as the Dem map already does, so they wouldn't be washed out by the rural Pub counties around them.
In Chicagoland Pubs might have seats in excess to their vote share, but downstate Dems would get seats beyond their share. Keep in mind that in order to guarantee minority representation, there will be a certain amount of Dem packing in Chicagoland. The VRA would keep it from rising to a point that overly favors the Pubs.
