And when I access the file via my normal route (mysite.com/sitemap.php) I also receive teh GZ file that it offers to download. The search engine recognizes those files as an archive and can extract the contents from them. The sitemap.php is a processing file that generates that GZ extension and sends it to the requestor.
Some scripts (or add-ons) will generate the text on the site and then send it - XenForo does it differently and sends the (perfectly acceptable) GZ archive that has the standard sitemap contents format in the compressed file.
Code:
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:image="http://www.google.com/schemas/sitemap-image/1.1">
<url><loc>https://thebent.bike/</loc></url>
<url><loc>https://thebent.bike/ams/category/diy-projects.2/</loc></url>
<url><loc>https://thebent.bike/ams/category/reviews.3/</loc></url>
<url><loc>https://thebent.bike/forums/the-rest-stop.21/</loc></url>
<url><loc>https://thebent.bike/forums/site-news-feedback.20/</loc></url>
<url><loc>https://thebent.bike/pages/privacy/</loc></url>
<url><loc>https://thebent.bike/forums/bike.26/</loc></url>
<url><loc>https://thebent.bike/forums/trike.27/</loc></url>
<url><loc>https://thebent.bike/forums/velomobile.33/</loc></url>
<url><loc>https://thebent.bike/forums/technical-assistance-zone.32/</loc></url>
<url><loc>https://thebent.bike/forums/ride-reports.34/</loc></url>
<url><loc>https://thebent.bike/forums/commuting.36/</loc></url>
<url><loc>https://thebent.bike/forums/touring-zone.35/</loc></url>
<url><loc>https://thebent.bike/forums/racing-zone.28/</loc></url>
<url><loc>https://thebent.bike/forums/equipment-talk.29/</loc></url>
<url><loc>https://thebent.bike/forums/electric-assist-powered.30/</loc></url>
<url><loc>https://thebent.bike/forums/for-sale-trade.31/</loc></url>
<url><loc>https://thebent.bike/forums/dfbikes/</loc></url>
<url><loc>https://thebent.bike/forums/trikes.16/</loc></url>
<url><loc>https://thebent.bike/downloads/catrike-owners-manual.1/</loc><lastmod>2015-12-02T06:13:11+00:00</lastmod></url>
<url><loc>https://thebent.bike/downloads/catrike-owners-manual.2/</loc><lastmod>2015-12-03T21:08:12+00:00</lastmod></url>
<url><loc>https://thebent.bike/tags/rally/</loc><lastmod>2015-11-19T18:59:03+00:00</lastmod></url>
<url><loc>https://thebent.bike/tags/austin/</loc><lastmod>2016-06-04T15:57:46+00:00</lastmod></url>
<url><loc>https://thebent.bike/tags/texas/</loc><lastmod>2015-11-19T18:59:03+00:00</lastmod></url>
<url><loc>https://thebent.bike/tags/559/</loc><lastmod>2015-11-21T02:51:53+00:00</lastmod></url>
<url><loc>https://thebent.bike/tags/catrike/</loc><lastmod>2015-11-21T02:51:53+00:00</lastmod></url>
<url><loc>https://thebent.bike/tags/owners-manual/</loc><lastmod>2015-12-03T21:08:12+00:00</lastmod></url>
<url><loc>https://thebent.bike/tags/easy-street/</loc><lastmod>2016-06-04T15:57:46+00:00</lastmod></url>
<url><loc>https://thebent.bike/tags/cargo/</loc><lastmod>2016-11-21T03:55:57+00:00</lastmod></url>
<url><loc>https://thebent.bike/tags/gear/</loc><lastmod>2016-11-21T03:55:57+00:00</lastmod></url>