I was always skeptical but the thing that made me concretely understand why these valuations aren't real is when I learned about liquidation preferences.

At least a 2x liquidation preference is standard for most VC investments (Y Combinator is an exception to this, I think). In simplified form, this means that regardless of what happens, the investors will get 2x of what they invested before anyone else in the company sees a cent. This means that even if giphy fails, the investors will always get a 2x return on their investment unless the company falls so far that it's no longer worth $24 million when it's sold off somehow.

EDIT: After being informed in responses to this, it appears that 1x preferences are now the standard. I apologize for any misinformation.