A small update on this. They struck a Russian reinforcement column - the Russians are claiming HIMARS (GMLRS) but who knows. The column was about 18 miles from the front line, which is within (extreme) artillery range (long range artillery doesn't want to be close to the frontline but even positioned far in the back could still hit that), but that could still mean US or German provided equipment. The video of the strikes from drone footage show impressively accurate fire whatever it was.
In terms of what I think is going on, for the last two years the Russians have been able to effectively concentrate their manpower advantage on the frontlines within Ukraine "knowing" that Ukraine is unable to attack across (primarily due to Western restrictions) while Ukraine has to spread its forces out because the Russians could, in theory (and in practice given the recent attack from Belgorod Oblast on Kharkiv), could attack anywhere from Belarus to Rostov. This exacerbates Russia's manpower advantage. Forcing Russia to defend its own territory could thin out its lines out and stop the slow grinding advances they've been making in the Donbas based on their meat grinder tactics. Can't do that as effectively if they haven't got the manpower.
A further interesting wrinkle is that the Sudzha, the primary town that the Ukrainians have captured is also home to the Gazprom facility that ships natural gas through Ukraine to the rest of Europe. The agreement that allows this oddity to take place expires in 2024 (and unsurprisingly Ukraine is unwilling to extend it) and Ukraine may be indirectly expediting its demise: forcing the Russians to retake their territory could risk destroying a strategically important facility while the Ukrainians can claim (not without reason) that the Russians, not they, are responsible for its destruction. Of course the Russians might not care too much since they'll be losing access to Europe at the end of the year, but even so, its always nice to place your enemy on the horns of a dilemma: continue to allow Ukrainians on Russian soil is obviously a nonstarter but destroying your own valuable strategic locations is also unpalatable.
Edit: and now there’s a report that the Ukrainians have attacked into Belgorod oblast taking the village of Poroz. If so, may it meet with as much success as their Kursk operation has so far.