Stargate Infinity occurs in an Alternate Universe in relation to the SG-1/Atlantis/Universe universe.Like the original movie. In "There But for the Grace of God", the alternate Carter says that alternate universes differ in their degrees of likeness to each other. It's not inconceivable that there's an alternate universe in which the Stargates have eight chevrons rather than nine, and the Ancients are Dragon-people.
Stargate Infinity was created by Martin Lloyd.It was produced as a Lighter and Softer spinoff of Wormhole X-Treme, and it was deliberately inaccurate to help maintain The Masquerade.
The 8-chevron Gates in Infinity are actually another variant of Stargate rather than a RetoolPerhaps constructed by a race which reverse-engineered the Stargate and resolved some of its hazards, such as the disintegration of objects in the path of the unstable vortex. Such a race might not have had a need for 9-chevron addresses or not have been aware of them.