Hyperbole.
Even if using advanced means, it is still a simulation running on computers, it'd be impossible for that to handle genuine infinity.
We also know for a fact the underworld is not only finite in size, but at it's largest only contained two planets within it because the cardinal system's only ever needed to generate that much in full detail.
Regarding Hyperbole, that is a dead author statement. Because it doesn't correlate with fiction, because in SAO there is a Milky Way galaxy and it is said to have a World End Wall which is said to have infinite size.
"I exhaled and surveyed the unthinkable scene before my eyes again. Stars of breathtaking density stretched in a band from top right to the bottom left of my vision. I suppose you could call it the Milky Way, but no matter how I lined up the potently glowing stars, I could not recognize a single familiar constellation from the real world. This was Underworld, all right.
𝙝, 𝙨𝙤𝙧𝙧𝙮 𝙍𝙤𝙣𝙮𝙚, 𝙗𝙚𝙞𝙣𝙜 𝙨𝙪𝙘𝙝 𝙖 𝙣𝙤𝙣𝙨𝙚𝙣𝙨𝙚...𝙪𝙝 𝙬𝙚𝙡𝙡...𝙗𝙖𝙨𝙞𝙘𝙖𝙡𝙡𝙮, 𝙩𝙝𝙚𝙧𝙚 𝙞𝙨 𝙣𝙤 '𝙀𝙣𝙙' 𝙞𝙣 𝙩𝙝𝙞𝙨 𝙬𝙤𝙧𝙡𝙙."
"𝙉𝙤 𝙚𝙣𝙙?"
𝙄𝙩 𝙬𝙖𝙨 𝙖 𝙘𝙤𝙣𝙘𝙚𝙥𝙩 𝙩𝙝𝙖𝙩 𝙬𝙖𝙨 𝙛𝙤𝙧𝙚𝙞𝙜𝙣 𝙩𝙤 𝙍𝙤𝙣𝙮𝙚.
𝙁𝙤𝙧 𝙍𝙤𝙣𝙮𝙚 𝙬𝙝𝙤 𝙬𝙖𝙨 𝙗𝙤𝙧𝙣 𝙖𝙣𝙙 𝙜𝙧𝙚𝙬 𝙪𝙥 𝙞𝙣 𝙉𝙤𝙧𝙩𝙝 𝘾𝙚𝙣𝙩𝙤𝙧𝙞𝙖, 𝙩𝙝𝙚 𝙜𝙞𝙜𝙖𝙣𝙩𝙞𝙘 𝙬𝙖𝙡𝙡 𝙩𝙝𝙖𝙩 𝙙𝙞𝙫𝙞𝙙𝙚𝙙 𝙩𝙝𝙚 𝙘𝙞𝙩𝙮 𝙞𝙣𝙩𝙤 4 𝙨𝙚𝙘𝙩𝙤𝙧𝙨 – 𝙖 𝙗𝙤𝙪𝙣𝙙𝙡𝙚𝙨𝙨 𝙬𝙖𝙡𝙡 – 𝙚𝙭𝙞𝙨𝙩𝙚𝙙 𝙛𝙧𝙤𝙢 𝙩𝙝𝙚 𝙗𝙚𝙜𝙞𝙣𝙣𝙞𝙣𝙜. 𝙃𝙚 𝙡𝙚𝙖𝙧𝙣𝙚𝙙 𝙢𝙖𝙣𝙮 𝙩𝙝𝙞𝙣𝙜𝙨 𝙖𝙗𝙤𝙪𝙩 𝙩𝙝𝙚 𝙉𝙤𝙧𝙡𝙖𝙣𝙜𝙖𝙧𝙩𝙝 𝙚𝙢𝙥𝙞𝙧𝙚 𝙖𝙣𝙙 𝙩𝙝𝙚 𝙃𝙪𝙢𝙖𝙣 𝙒𝙤𝙧𝙡𝙙 𝙨𝙞𝙢𝙪𝙡𝙩𝙖𝙣𝙚𝙤𝙪𝙨𝙡𝙮 𝙧𝙪𝙡𝙚𝙙 𝙗𝙮 3 𝙚𝙢𝙥𝙚𝙧𝙤𝙧𝙨. 𝙃𝙚 𝙝𝙖𝙙 𝙤𝙣𝙘𝙚 𝙩𝙝𝙤𝙪𝙜𝙝𝙩 𝙖𝙗𝙤𝙪𝙩 𝙩𝙝𝙚 𝙚𝙣𝙙𝙡𝙚𝙨𝙨 𝙢𝙤𝙪𝙣𝙩𝙖𝙞𝙣𝙨 𝙨𝙪𝙧𝙧𝙤𝙪𝙣𝙙𝙞𝙣𝙜 𝙩𝙝𝙚 𝙃𝙪𝙢𝙖𝙣 𝙒𝙤𝙧𝙡𝙙 𝙖𝙣𝙙 𝙩𝙝𝙚 𝙛𝙤𝙧𝙗𝙞𝙙𝙙𝙚𝙣 𝙩𝙚𝙧𝙧𝙞𝙩𝙤𝙧𝙞𝙚𝙨 𝙩𝙝𝙖𝙩 𝙝𝙖𝙙 𝙨𝙥𝙧𝙚𝙖𝙙 𝙬𝙞𝙙𝙚𝙡𝙮 𝙚𝙫𝙚𝙧 𝙨𝙞𝙣𝙘𝙚 𝙝𝙚 𝙚𝙣𝙩𝙚𝙧𝙚𝙙 𝙨𝙘𝙝𝙤𝙤𝙡 𝙖𝙩 𝙩𝙝𝙚 𝙖𝙜𝙚 𝙤𝙛 8.
𝙀𝙫𝙚𝙣 𝙩𝙝𝙤𝙪𝙜𝙝 𝙝𝙞𝙨 𝙩𝙚𝙖𝙘𝙝𝙚𝙧 𝙙𝙞𝙙𝙣'𝙩 𝙩𝙚𝙖𝙘𝙝 𝙨𝙥𝙚𝙘𝙞𝙛𝙞𝙘𝙖𝙡𝙡𝙮 𝙖𝙗𝙤𝙪𝙩 𝙩𝙝𝙚 𝘿𝙖𝙧𝙠 𝙏𝙚𝙧𝙧𝙞𝙩𝙤𝙧𝙮 𝙩𝙝𝙞𝙣𝙠𝙞𝙣𝙜 𝙖𝙗𝙤𝙪𝙩 𝙞𝙩, 𝙞𝙩 𝙛𝙚𝙡𝙩 𝙙𝙤𝙪𝙗𝙩𝙛𝙪𝙡 𝙩𝙤 𝙨𝙥𝙚𝙖𝙠 𝙖𝙗𝙤𝙪𝙩 𝙞𝙩 𝙖𝙣𝙙
𝙩𝙝𝙖𝙩'𝙨 𝙬𝙝𝙖𝙩 𝙞𝙩'𝙨 𝙘𝙖𝙡𝙡𝙚𝙙
𝙏𝙝𝙚 '𝙒𝙤𝙧𝙡𝙙'𝙨 𝙀𝙣𝙙 𝙒𝙖𝙡𝙡'—𝙝𝙚 𝙝𝙖𝙙 𝙤𝙣𝙡𝙮 𝙡𝙚𝙖𝙧𝙣𝙚𝙙 𝙖𝙗𝙤𝙪𝙩 𝙞𝙩 𝙛𝙧𝙤𝙢 𝙩𝙝𝙚 𝙛𝙤𝙧𝙢𝙚𝙧 𝙝𝙪𝙢𝙖𝙣 𝙬𝙤𝙧𝙡𝙙 𝙙𝙚𝙛𝙚𝙣𝙨𝙚 𝙛𝙤𝙧𝙘𝙚 𝙬𝙞𝙩𝙝 𝙏𝙞𝙚𝙯𝙚 𝙪𝙥𝙤𝙣 𝙖𝙧𝙧𝙞𝙫𝙞𝙣𝙜 𝙖𝙩 𝙩𝙝𝙚 𝙂𝙧𝙚𝙖𝙩 𝙂𝙖𝙩𝙚 𝙤𝙛 𝙩𝙝𝙚 𝙚𝙖𝙨𝙩 𝙚𝙣𝙘𝙖𝙢𝙥𝙢𝙚𝙣𝙩. 𝙄𝙣 𝙤𝙩𝙝𝙚𝙧 𝙬𝙤𝙧𝙙𝙨, 𝙩𝙝𝙖𝙩 '𝙒𝙖𝙡𝙡' 𝙝𝙖𝙙 𝙖𝙡𝙬𝙖𝙮𝙨 𝙚𝙭𝙞𝙨𝙩𝙚𝙙 𝙞𝙣 𝙩𝙝𝙞𝙨 𝙬𝙤𝙧𝙡𝙙. 𝙀𝙫𝙚𝙣 𝙞𝙛 𝙮𝙤𝙪 𝙥𝙖𝙨𝙨, 𝙩𝙝𝙚𝙧𝙚 𝙬𝙞𝙡𝙡 𝙙𝙚𝙛𝙞𝙣𝙞𝙩𝙚𝙡𝙮 𝙗𝙚 𝙖𝙣𝙤𝙩𝙝𝙚𝙧 𝙬𝙖𝙡𝙡 𝙩𝙝𝙖𝙩 𝙖𝙥𝙥𝙚𝙖𝙧𝙨. 𝙃𝙚 𝙗𝙚𝙡𝙞𝙚𝙫𝙚𝙙 𝙩𝙝𝙖𝙩 𝙤𝙣𝙚 𝙙𝙖𝙮 𝙨𝙤𝙢𝙚𝙤𝙣𝙚 𝙬𝙤𝙪𝙡𝙙 𝙗𝙚 𝙖𝙗𝙡𝙚 𝙩𝙤 𝙥𝙖𝙨𝙨 𝙩𝙝𝙧𝙤𝙪𝙜𝙝 𝙩𝙝𝙖𝙩 𝙒𝙤𝙧𝙡𝙙'𝙨 𝙀𝙣𝙙 𝙒𝙖𝙡𝙡 𝙥𝙚𝙧𝙛𝙚𝙘𝙩𝙡𝙮.
"...𝙬𝙚𝙡𝙡, 𝙡𝙞𝙠𝙚...𝙮𝙤𝙪 𝙢𝙚𝙖𝙣, 𝙤𝙣 𝙩𝙝𝙚 𝙤𝙩𝙝𝙚𝙧 𝙨𝙞𝙙𝙚 𝙤𝙛 𝙩𝙝𝙞𝙨 𝙒𝙤𝙧𝙡𝙙 𝙀𝙣𝙙 𝙒𝙖𝙡𝙡, 𝙞𝙨 𝙞𝙩 𝙡𝙞𝙠𝙚 𝙩𝙝𝙚 𝙃𝙪𝙢𝙖𝙣 𝙒𝙤𝙧𝙡𝙙 𝙖𝙣𝙙 𝙩𝙝𝙚 𝘿𝙖𝙧𝙠 𝙏𝙚𝙧𝙧𝙞𝙩𝙤𝙧𝙮 𝙖𝙜𝙖𝙞𝙣? 𝙂𝙧𝙖𝙨𝙨𝙡𝙖𝙣𝙙𝙨, 𝙛𝙤𝙧𝙚𝙨𝙩𝙨, 𝙛𝙞𝙚𝙡𝙙𝙨 𝙖𝙨 𝙛𝙖𝙧 𝙖𝙨 𝙩𝙝𝙚 𝙚𝙮𝙚 𝙘𝙖𝙣 𝙨𝙚𝙚?"