It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.potions.IPotionType;

You can get such an object through the use of the PotionType Bracket handler

ZenScript
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<potiontype:minecraft:long_strength>;

ZenGetter	类型
ZenGetter ​	类型 ​
ZenGetter effects	类型 List<IPotionEffect>

This method does the same as the PotionType Bracket Handler.

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//fromString(String potiontype);
IPotionType.fromString("minecraft:mundane");