Maybe they changed it. UMoveSceneFloatSection has a GetChannel() member and in the channel (FMovieSceneFloatChannel) has Evaluate(FFrameTime InTime, float& OutValue).
If you have the float section, you maybe could evaluate it with
const FMovieSceneFloatChannel& Channel = FloatSection->GetChannel();
int32 FrameNumber = 0; // the frame to evaluate the curve at
float Value;
bool Success = Channel.Evaluate(FFrameTime(FrameNumber), Value);
if (Success)
{
// successfully evaluated. Value has the curve value.
}