The link you sent explains how to compile SageMath from its source code. This is more or less what is described in the manual at https://doc.sagemath.org/html/en/installation/source.html.
Most likely, sage is installed as as a python virtual environment. You can still run Python in this environment and then use from sage.all import *.
Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.