It's possible because the URL is hosted on a high-authority domain like Sketchfab that Google's crawlers visit every few minutes. These sites have an established "crawl budget" priority. When you post there, the page gets discovered and indexed in the next crawl cycle, which can be within minutes.
The more critical factor is getting ranking signals immediately after indexing. They likely use tiered link blasts with anchor text from already-indexed pages (like social bookmarks or web 2.0s) pointing to the new Sketchfab URL. This tells Google the page is already "important," so it gets processed faster in the index.
Another method is leveraging Google's Indexing API, originally for job listings but sometimes abused. Tools exist that mimic the API's structure, tricking Google into thinking the new page is time-sensitive content that needs immediate crawling. This is risky and can lead to penalties if detected.
For more sustainable speed, focus on sites with real-time publishing (like Blogger, WordPress.com, or high-traffic forums). New posts there often get indexed within 5-10 minutes because Google trusts the domain's update frequency. Posting during peak crawl times (often correlating with the site's primary audience timezone) can also shave off minutes.