Read below for the actual license but in summary you can use the illustrations in any project, commercial or personal without attribution or any cost. Just don’t try to replicate PassionHacks, redistribute in packs the illustrations or create integrations for it.
If you are working on something and want to use illustrations to improve its appearance, modified or not, without the need for attribution or cost, you are good to go. If you find PassionHacks or its illustrations to be in the center of what you are doing (e.g. sell or re-distribute one/some of them, add them in an app), then you probably should not proceed.
Full license text
Copyright 2021 PassionHacks
All images, assets and vectors published on PassionHacks can be used for free. You can use them for noncommercial and commercial purposes. You do not need to ask permission from or provide credit to the creator or PassionHacks, but we always appreciate if you do.
More precisely, PassionHacks grants you an nonexclusive, worldwide copyright license to download, copy, modify, distribute, perform, and use the assets provided from PassionHacks for free, including for commercial purposes, without permission from or attributing the creator or PassionHacks. This license does not include the right to compile assets, vectors or images from PassionHacks to replicate a similar or competing service, in any form or distribute the assets in packs or otherwise. This extends to automated and non-automated ways to link, embed.