If you want to skip Business Insider here, the “think tank” they’re referring to is the Institute of War. All BI is doing is summarizing the July 7th ISW conflict analysis, which you can read directly from the “think tank” here: https://www.understandingwar.org/backgrounder/russian-offensive-campaign-assessment-july-7-2023
To summarize what the report says, it seems like most of the Eastern Military District (EMD) is at the southern/eastern frontlines with elements from the southern md as well. This translates as a vacuum of support from rear lines that could be essential in the event that part of the line needs to do a tactical retreat.
The report also makes the common confirmation that Russia’s lines of defense are extensive and well prepared, and it will take a huge effort to pierce them, however if Ukraine is successful in doing so, lack of reinforcements could possibly make the line crumble.
