The same excess of energy during peak renewable production exists in California, and parts of Europe. So this problem is more than just Hawaii's geology. There are costs in electrolysis, compressions, decompression, and conversion back to electricity that are just being hand-waved away.
Can you point me to a developer that's actually offering to build hydrogen electric grid storage at $1/KWh? As in, if I give them $1 million they will build 1 GWh of hydrogen electric storage for me. Are there any enterprises actually willing to provide grid storage at this cost? If so, please point them my way. I'll make a massive amount of money. But I doubt I'll have anyone taking this offer.
The storage costs your citing are absolutely incredible. As in, I genuinely do not believe them. You're claiming that the entirety of the US's grid storage (which cost billions of dollars to build, mostly in the form of hydroelectric storage) can be matched by only $20 million in hydrogen storage. This is a cost estimate totally disconnected from reality. Until enterprises are actually building hydrogen electric grid storage for $1/KWh then this figure is meaningless.
If not then what's your explanation as to why people are missing out on the opportunity to become billionaires or trillionaires by construction hydrogen electric storage? Bill Gates alone could build enough storage for 24 hours of the USA's electricity consumption with only 10% of his net worth. This would be a rounding error on the national budget.