Hi everybody! Finally got it working! 
I went several steps back looking for what could be the problem. At the end it was a misconfiguration on the site.php blueprint where it was forcing the default one (Title + Text), just as @texnixe explained in this post many days ago → Bydefault redirected to default template
Since the site blueprint was forcing the content created in the “Projects” page, all the text files were named “default.txt”, so I changed them to their corresponding name and the thumbnails showed up, in this case while testing with the snippet that @Thiousi posted here, since the correct blueprint and template for projects were being applied.
So, the lesson I learned with this issue: Always check the site.php —and the .txt filenames—, especially if you are working with the Kirby starterkit. (I am leaving it posted in case someone gets the same problem in the future).
In the end it’s kind of a rookie mistake, but a setting that many people may ignore.
Thank you all for your time and help 
!!!
Cheers!