You're right, TASS is a Russian state press agency transmitting official propaganda but what this is an example of is Russians admitting their own loss. It rarely happens in this war, which makes this all the more interesting.
Because the fact that it is relayed by a pro-putin newspaper is significant.
This was is also an information war, so analysing who reports the news is almost as important as what is reported.
TASS is a news agency, like Reuters or AP, not a newspaper.
While it's significant, it's also propaganda. At the very least, WP has a policy to maintain a neutral point of view and to only put things on the site which can be reliably sourced.
We should be sensitive to the power of a site like this in relaying harmful propaganda.