That is because of the way the Acura was balanced. The authors balanced the Acrua against the S397 Caddy but their post doesn’t say which form. When we had the Caddy locked to DPi mode with was not a correct match, however opening up the Hypercar setting made it competitive with the Acura so that is what the Acura was balanced against, the Hypercar setting.
This I believe was done for groups who wanted to do IMSA style events such as us where the DPis are faster than the LMP2 in real life. However the real DPi class was comprised of modified LMP2 rules so IMSA slowed the LMP2 cars down, which the majority of leagues do not have the ability to do in RF2, and S397 corrected that by upping the abilities on the Cadillac by adding the second setting