When this story takes place (date given is 1769), dinosaurs had not yet been discovered as an extinct species. It would take another 80 years, according to “Early Dinosaur Discoveries in North America”, where it is said: “It is generally accepted that the first discovery of dinosaur remains in North America was made in 1854 by Ferdinand Vandiveer Hayden during his exploration of the upper Missouri River.” To be fair to the Tomahawk writer, one caption mentions dinosaurs, but Tomahawk never calls them that. Still, he does know that the giants are critters of another time.
It reminds me of the basic premise of Turok, Son of Stone. Turok and Andar were two pre-Columbian Indians who stumbled into the lost valley of dinosaurs, and couldn't find their way out. Who knows? Maybe it was the same valley.
From Star Spangled Comics #83 (1948). Drawn by Fred Ray.
**********In later years Tomahawk encountered all kinds of crazy stuff: aliens, dinosaurs, supernatural creatures, etc. You can find an especially oddball Tomahawk story from the mid-'60s which I featured in 2010. Just click the picture.