InDesign Color 400 ppi Images Export Blurry so are Unfit for my Novel's Cover Design

You probably really need to be making a print PDF. But printers are not generic - you must find out their needs and meet them. 

The images in your PDF are 72ppi—458 x 152 pixels:

To add to what Derek has mentioned

You cannot just upscale images to 400ppi - it doesn't work like that.

Your image needs to be originally 400ppi at the dimensions you require.

You would probably need to start with a higher resolution image. 

Can you share a link to the image on Unsplash that you got?

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Typically 400ppi is quite large and not needed for print and you can usually be anywhere from 225 to 300ppi depending on the output machines for the book. Typically people aim for 300ppi.

I wrote an article here that might interest you to shed some light why images look blurry.

https://creativepro.com/high-res-image-look-low-res/#:~:text=By%20default%2C%20InDesign%20is%20set,and%20click%20on%20Display%20Performance.

It's not the resolution that the image is in Photoshop that will be in your placed image in InDesign but the size you place it, so if you enlarge it, for example, the resolution will be lower. You can check the Effective PPI (which is the actual InDesign image resolution) in the Links panel.

(Vector images such as those created in Adobe Illustrator have no resolution.)

Also, ensure you're viewing the InDesign document at the best screen reproduction:

View > Display Performance > High Quality Display.