The MIT license (i.e., the terms under which Steve Donovan offers gentle-intro) says
Permission is hereby granted [...] to deal in the Software without restriction, including [...] the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software [...] subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
Since slashtutorial.com does indeed include the copyright notice and license text (when you click on the "License" footer link), it may therefore "use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software".
Whether or not you consider this "us[ing] open source content as your own" is a matter of semantics -- certainly the author of slashtutorial.com does not make a claim that it is his or her own, since they clearly attribute Steve Donovan as the copyright holder and author.