Nvm, I have figured it out.
The elseif part is there for items that by default do NOT have weight assigned (stack inserter, landfill, most of tiles, centrifuge, some materials besides others). Problem is, the elseif changes items without assigned weight to have weight so that exactly one stack is 1000kg. But this unfortunately makes it that rocket can carry less items in than in vanilla.
Example: Electronic circuit has no weight assigned. elseif in the mod looks at the stack size (200) and uses it for the weight calculation - 1000/200/1 * kg = 5kg, so your rocket can then carry exactly 200 of them.
I do not know how to tell the mod, that if the default rocket carry value is higher than stack, to ignore the mod calculation alltogether. I wonder if there is some internal value preprogrammed for certain items without weight (like the electronic circuit for example) that we could use to compare the elseif value to. Otherwise only way how to solve this that I see is to manually add weight to all items that currently do not have weight assigned.