KOSPI closes down 2.70%, and two stories are fighting over the same decline

Start with the numbers that are not in dispute, because almost every intraday figure in circulation was. On the Korea Exchange the KOSPI closed at 6,889.74, down 191.18 points or 2.70%. Samsung Electronics closed at ₩272,000, down 4.73%. SK Hynix closed at ₩1,777,000, down 4.56%. SK Square, the holding company above SK Hynix, closed at ₩1,099,000, down 7.65%. Elsewhere in Asia, Kioxia fell 2.5%, SMIC 3.6% and Hua Hong 4.8%. Those are exchange closes; the roundups' intraday prints disagreed with each other and with the closes, and are not used here.

The first explanation is OpenAI. A pause on frontier training revives the worry that a slower pace of AI development eventually curbs demand for advanced chips, servers and data-centre capacity, and memory names fall hardest because training clusters are where high-bandwidth memory volume lives. The chain worth naming precisely is that no company has announced a change to an order, a capex plan or a delivery schedule. This is sentiment reacting to a possibility.

The second is Solidigm. Reuters reported on Friday that SK Hynix's US NAND and SSD subsidiary held pitch meetings with banks for an IPO as early as 2027 that could value it at up to $150bn and raise $15bn. A valuation is not cash raised, and neither figure is a company figure — both are attributed to unnamed sources describing early-stage meetings. SK Hynix's own statement is deliberately narrow: “Solidigm is reviewing various options to strengthen its business competitiveness, but no specific plans have been confirmed at this time.” Three outlets put the valuation at $100bn; the originating report says $150bn, so the lower figure is a derivative error rather than a second opinion, and it is not carried.

Now put the two against each other. Samsung has no Solidigm exposure and fell 4.73%, which argues the sector-wide input did most of the work. But SK Square fell roughly three percentage points more than the asset it holds, which argues the Solidigm report did some of it — consistent with a market pricing the risk that value migrates into a separately listed US vehicle rather than being distributed up the chain. Anyone attributing the whole of either move to a single cause is guessing.

• Reported KOSPI closed at 6,889.74 on 28 September, down 191.18 points or 2.70%; Samsung Electronics −4.73%, SK Hynix −4.56%, SK Square −7.65%. Yahoo Finance quote pages
• Reported Solidigm's potential US IPO could value it at up to $150bn and raise $15bn, as early as 2027; three outlets reported $100bn, which contradicts the originating report. Reuters via Investing.com
• Claimed SK Hynix: “Solidigm is reviewing various options to strengthen its business competitiveness, but no specific plans have been confirmed at this time.” Reuters via Investing.com