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.

I wonder if part of the reticence to give Ukraine a firm commitment for joining NATO is they expect it to be a negotiating chip with Russia in future peace talks. Pausing Ukraine's NATO membership bid might be the way to give Russia an out without territory concessions...