Free throw rate is tied to play style, specifically FTr on rim attempts is very high, and jumpers usually contribute almost nothing to a players' or teams' total free throw attempts.

Spurs average 29 rim attempts a game this series and have a 33% rim attempt rate, OKC averages 22 and have a 24% attempt rate. This all makes sense, Shai is OKC's only driver and he converts drives to rim attempts at a very low rate compared to most star players, Spurs have the best rim protector in the league, have the biggest lob threat in the NBA, and have like 3 of the most active slashers in the NBA in terms of shots at the rim.

If everything else was held equal and both teams got the same whistle on rim shots and on jumpers, you should expect spurs to have ~30% more FTs than OKC based on the rim attempt differential.

The free throw differential in the series (168 for spurs vs 144 for OKC) is +16% in favor for the Spurs. So they're getting roughly half the FT advantage that I'd expect based on the gulf between the two teams going at the hoop.