I think the main question about Corgi is: are they underpricing risk so severely that they go bust? And honestly, we have no idea.
For all we know startups are buying overpriced insurance from Corgi because they have a better brand and are easier to deal with than Berkshire's army of underwriters.
Though it's also worth noting that the main reasons startups buy insurance is not because they want insurance, but because enterprise customers demand insurance. Which is to say, it's not out of the realm of possibility that funded startups are not actually that price sensitive, because they just want to get the deal signed and move on.
We got our insurance elsewhere because we're a little older, so I have no actual opinion of Corgi, but there's a lot of stuff that enterprise customers demand that is driven by some compliance checklist. Delve took this to an extreme, but directionally, they were providing the service customers wanted, and at least in the insurance market, you can just pay more to paper over your problems rather than addressing the core risks in a way where there is no fraud. We pay for random shit we don't need that delivers no value for enterprise customers to tick boxes, for all I know Corgi fills the same need.