I took that as they have encountered other species before, just nothing like the creature from the Alien franchise. The term xenomorph is just used to describe any creature not found on Earth, aka aliens.
This is backed by an earlier comment made during Ripley’s hearing with the company:
Insurance attorney: Are there any species like this hostile organism on LV-426? ECA representative: No, it’s a rock. No indigenous life.
Which would indicate they have encountered other aliens, just not this Alien.
The creature was named both Alien and Xenomorph because that’s what it is, but came to define the names and took “ownership” of the names, if you will.