This seat had been GOP for at least 50 years, before the last election.
I see that further in the article: “The seat was a key victory for Republicans. District 36, which flipped blue in 2018, gave Democrats a supermajority in the Senate. The seat came up for special election following the resignation of former state Sen. Alex Kasser.”
So it was a gain, but maybe not as huge as it is being made out to be.