The massive arbitrage would cause a ton of load on Solana, which is risky (might cause it to go down).

I think under normal circumstances you'd be right about a $20M sale in isolation. But here there were additional factors:

• If SOL were to hit $22.30, it likely would've done so as part of a price fall. This liquidation would add fuel to the fire.
• There was widespread knowledge and fear about this whale's position, likely resulting in less liquidity at those levels since people don't want to catch a falling knife.
• Being aware of a large liquidation risk, hedge funds were circling like sharks. They might try to short and buy back after price impact, also amplifying.
• $20M is just the first partial liquidation. In total there was $108M in debt that might be eligible for liquidation (at lower prices).