Not that it is illusion. Kṛṣṇa is illusion or Kṛṣṇaloka is illusion. No, it is not illusion; it is fact. Goloka-nāmni nija-dhāmni tale ca tasya (Bs. 5.43). In the Vedic literature we find description of the planet of Goloka Vṛndāvana, the abode of Kṛṣṇa. It is stated, goloka-nāmni nija-dhāmni. Nija-dhāmni. Nija-dhāmni means the personal abode. Kṛṣṇa is person. God is person.
Just like governor. The governor is a person, and he has got his personal place, the government house. Similarly, the Supreme Lord.... Brahmeti paramātmeti bhagavān iti (SB 1.2.11). He is person, and He has got His abode. That is stated in the Brahma-saṁhitā.