Ground screws and helical foundations come with engineering documents for permitting that indicate the pullout strength as a function of torque. So they can use a large torque wrench when they're installed to verify the pullout strength. Did your system come with engineering documents for permitting? Those documents should specify the size of the pipe selected as a function of soil type, frost depth, bearing load, uplift and lateral loading.
Besides uplift there is also issues with load bearing and frost heave. Ground screws and helical foundations have to extend 3-4' below the frost line to resist the force of an ice lens trying to lfit it out of the ground. If one vertical support heaves and the others don't, that would damage the mounting system and the panels. We have a 42" frost depth so my ground screws had to be 7' in the ground. They also are closed on the ends so they can take the bearing load, particularly in snow areas. We have to design for 40psf snow load on top of the static load. In my case that was 20,000# distributed over 10 ground screws so 2000# each. An open pipe only has a direct bearing surface equal to the cross sectional area of the wall thickness. So it's dependent on the soil friction to keep it from sinking.
I have seen piles used for buildings but not open pipes. The piles require core samples for a soil engineering analysis to determine the depth and surface area of the piles. Even small buildings have piles that extend 15' or more into the ground. For larger buildings the ground is drilled before the piles are driven and then grout is injected to bond the pile to the sides of the soil. There are other techniques for piles, but they all require an engineering analysis of the specific conditions.