If this is part of the license then it is not open source. It's just free under their license agreement. Most of the other icon libraries listed above are licensed under the MIT License which is the most permissive license.
This basically means that you can not use these icons in a product that you will further sell, such as a website template in comparison with the other libraries listed above where you can do whatever your heart wishes :)
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It is not open source, you can only use it for personal websites for instance. Here's an example from their license agreement:
"Licensee may not sell, redistribute, sublicense, share or otherwise transfer the Work to other people
or entities."
Although their icons look great it's not open source :(
iconmonstr.com/license/
But the license also says
I believe that means you can use them commercially, but you don't own them?
If this is part of the license then it is not open source. It's just free under their license agreement. Most of the other icon libraries listed above are licensed under the MIT License which is the most permissive license.
Okay thanks for clarifying
This basically means that you can not use these icons in a product that you will further sell, such as a website template in comparison with the other libraries listed above where you can do whatever your heart wishes :)