[Google Pixel Questions & Answers] metricusa:
OK I finally figured it out.

To clarify, the purpose was to burn a factory image and start from scratch. (no sideload). All without having to install Android studio or the SDK manager or whatever. This is a guide for Linux but I believe this method works in Windows as well with appropriate syntax.

1. First rule is to NEVER try to use a version of fastboot other than the one "prescribed" for the image you want to burn. To learn what version to use check Google's SKD Platform release Notes here: https://developer.android.com/studio/releases/platforms.html (my major mistake was to think that ubuntu would have the latest fastboot available in its repositories. Don't even try!)

2. Find the SDK Platform Tools version indicated on the above page. (In my case: Android SDK Platform-Tools 25.0.1 as I wanted to burn Android 7.1.1.). Google does not make them available AFAIK but other people do. I found them here: http://androidsdkoffline.blogspot.com/p/android-sdk-platform-tools.html OR do a Google Search.

3. Unpack the platform tools and the Factory image you wish to burn. Then copy fastboot from the Platform Tools directory to the image directory.

4. run fastboot from inside the directory. You can do this in at least 2 ways:

4a- manually run one-by-one the commands listed in the flash-all.sh script making sure you add sudo ./ in front of each fastboot command

4b- edit flash-all.sh and add sudo ./ in front of each fastboot command. Then run the flash-all script by typing ./flash-all.sh

There are of course more ways but I found this to be the simplest method as it doesn't involve installing any package and/or modifying your system variables. Plus, after you are done you can simply delete all files and directories you created.

Hope this helps other people too.

A final observation: It appears to me that starting with pixel the phones have 2 bootable sectors "slot a" and "slot b". See output below:

Creating filesystem with parameters:

Size: 26663190528

Block size: 4096

Blocks per group: 32768

Inodes per group: 8192

Inode size: 256

Journal blocks: 32768

Label:

Blocks: 6509568

Block groups: 199

Reserved block group size: 1024

Created filesystem with 11/1630208 inodes and 146354/6509568 blocks

--------------------------------------------

Bootloader Version...: 8996-012001-1610061102

Baseband Version.....: 8996-012511-1611190200

Serial Number........: FAxxxx

--------------------------------------------

checking product...

OKAY [ 0.050s]

checking version-bootloader...

OKAY [ 0.051s]

checking version-baseband...

OKAY [ 0.050s]

sending 'boot_a' (24821 KB)...

OKAY [ 0.642s]

writing 'boot_a'...

OKAY [ 0.299s]

erasing 'system_a'...

OKAY [ 0.415s]

sending sparse 'system_a' 1/3 (516463 KB)...

OKAY [ 12.799s]

writing 'system_a' 1/3...

OKAY [ 5.009s]

sending sparse 'system_a' 2/3 (520623 KB)...

OKAY [ 14.101s]

writing 'system_a' 2/3...

OKAY [ 7.052s]

sending sparse 'system_a' 3/3 (517483 KB)...

OKAY [ 12.818s]

writing 'system_a' 3/3...

OKAY [ 5.449s]

erasing 'system_b'...

OKAY [ 0.354s]

sending sparse 'system_b' 1/4 (522362 KB)...

OKAY [ 12.856s]

writing 'system_b' 1/4...

OKAY [ 3.890s]

sending sparse 'system_b' 2/4 (514082 KB)...

OKAY [ 12.758s]

writing 'system_b' 2/4...

OKAY [ 4.302s]

sending sparse 'system_b' 3/4 (524026 KB)...

OKAY [ 12.967s]

writing 'system_b' 3/4...

OKAY [ 3.839s]

sending sparse 'system_b' 4/4 (88446 KB)...

OKAY [ 2.260s]

writing 'system_b' 4/4...

OKAY [ 0.842s]

erasing 'vendor_a'...

OKAY [ 0.153s]

sending 'vendor_a' (235186 KB)...

OKAY [ 5.804s]

writing 'vendor_a'...

OKAY [ 3.846s]

Setting current slot to 'a'...

OKAY [ 0.099s]

erasing 'userdata'...

OKAY [ 1.806s]

sending 'userdata' (138957 KB)...

OKAY [ 3.451s]

writing 'userdata'...

OKAY [ 1.301s]

rebooting...

finished. total time: 129.610s