Knowing the XML I think it's unavoidable, rather than intended
</entity>
<entity name="AntiGravitateBlock"
display_name="AntiGravitateBlock"
category="Obstacle"
special_object_type="AntiGravitateBlock"
max_health="150"
>
<hitbox radius="96" />
<system type="AntiGravitateBlock" offset="0,0" />
</entity>
<system name="AntiGravitateBlock"
category="DirectUseSystem"
firing_timing="AllTheTime"
only_fires_on_death="true"
fire_rate="2"
image_name="Invisible"
>
<effect timing="OnUse" type="SpawnEnemy" times_to_check="2" chance_out_of_100_to_happen="50" related_entities="AntiGravitateShards100" magnitude="10" second_magnitude="50" third_magnitude="90" />
<effect timing="OnUse" type="SpawnEnemy" times_to_check="3" chance_out_of_100_to_happen="50" related_entities="AntiGravitateShards70" magnitude="30" second_magnitude="70" third_magnitude="120" />
<effect timing="OnUse" type="SpawnEnemy" times_to_check="4" chance_out_of_100_to_happen="50" related_entities="AntiGravitateShards50" magnitude="50" second_magnitude="90" third_magnitude="150" />
<effect timing="OnUse" type="SpawnEnemy" times_to_check="7" chance_out_of_100_to_happen="50" related_entities="AntiGravitateShards20" magnitude="70" second_magnitude="120" third_magnitude="220" />
</system>
It's needing to use spawn enemy (which requires the thing spawned to be one), as it's coded to use an effect.
It is impossible to do things that vary the number spawned without using an effect, or at least very, very hard.
You could use the effect to add system and then create tons of systems that fire one bullet pattern (spawning the entity) then die. Which is messy as hell, as you then need something to attach those systems to (can't use the dead parent entity).