The linker (google in this case) could rewrite the link to use a redirector if they choose. If Javascript is off, AMP and thus Signed Exchanges are disabled on Google search results anyway.

You misunderstand the 8 second CSS animation in the AMP boilerplate. Here's the code (simplified):

  <style>
    body { animation:-amp-start 8s steps(1,end) 0s 1 normal both}
    @keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}
  </style>
  <noscript>
    <style amp-boilerplate>
      body{animation:none}
    </style>
  </noscript>

See the noscript section: if javascript is disabled, the CSS displays the body immediately. If Javascript is enabled, but for some reason the AMP javascript fails to load, after 8 seconds, the page is displayed anyway. The page is probably somewhat broken without the javascript loading, but the 8s is a fallback, not code to slow down non-javascript browsers.