That's what the evidence seems to point to... or that Gravity was somehow attenuated by another means. For animals of these masses to have stood up, lifted their heads, walked, mated, etc., the implication is that Earth's gravity should be an apparent 1/4 to 1/3 G... not the 1 G we experience today.
Another possibility, an outside one, is that dinosaur and other mega-fauna muscles were three to four times stronger per cross-sectional square centimeter than modern animal muscles. . . and that dinosaurs were far more efficient at converting food into calories to drive those super muscles.
I’d think that maybe the earth may have been spinning faster.
Gravity is one issue, the amount of oxygen in the air is another. The lungs of these animals is nowhere near a workable dimension in order to feed such a large muscle mass. The only explanation is that there was simply more oxygen then, in the same volume of air.