I can confirm that they do not parse low ranking JavaScript sites, but they will parse higher ranking JavaScript sites.

Bots aside (Twitter and Slack still don't parse JS), it is still in your best interest to server-side render or pre-render a JavaScript site. The time-to-first-paint difference always starts small, but as you add features the JS apps become so bloated you'd be better off starting over with a server-side rendered HTML app.

(I am the creator of https://www.prerender.cloud/)