The tag of "in-game purchases" has been used by the ESRB to describe DLC expansions for games in the past, so it could be that Starfield is actually set to receive post-launch content instead of having a more conventional in-game store.
Considering Bethesda has an extensive history of post-launch expansions, dating all the way back to Fallout 3 and The Elder Scrolls 4: Oblivion, we wouldn't be surprised if Starfield also grows after launch. But Bethesda also has a history of DLC, to be fair, with its most notorious being Oblivion's iconic horse armor.
There's also a third possibility for this tag: Bethesda's Creation Club. The developer offers this in Skyrim and Fallout 4 as a way of rolling out both developer- and fan-made content for players to download at no extra cost. Although downloading Creation Club content wouldn't cost you anything, it'd still technically be an in-game purchase due to the fact you're downloading something from within Starfield itself.
Starfield could have microtransactions, DLC expansions, Creation Club content, or all three. Just don't take this ESRB rating to mean that the new Bethesda game will absolutely have microtransactions, either when it launches on September 6, or sometime after release.
Before its launch, we've got the Starfield Direct to look forward to on June 11, where Bethesda and Xbox have pledged to reveal "much, much more" about the sci-fi RPG at the big showcase.
Xbox's head recently reassured Starfield fans about the upcoming game after Redfall debuted to disappointing reviews and player reception earlier this month.