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.
How much of a reliable source is Business Insider as opposed to... say something like Newsweek? Do they publish hearsay and wishful thinking, or do they corroborate with sources?
They are just using the Institute for the Study of War as their source - and they are the main source used by most western media articles on the war in Ukraine. The ISW basically analyze all the available open source data (videos, Russian milbloggers, etc.) to compile daily reports and map of the front lines.
They're quite good, I check in on them directly regularly. After a while you notice the news articles are just regurgitating their daily reports.