Hi,
In the example we are using categories and tags for the filters – it is detailed in steps 2-4 in the tutorial.
You can do that same with custom fields too. For those I recommend checking the custom field filters section. Those are probably even a bit simpler to setup and easier to use as taxonomy terms.
I generally recommend using taxonomies for reusable data – for example languages, categories etc.. You can assign the same language to multiple posts, and in that case it is more efficient to use taxonomies, as the storage is simpler. For more random or numeric or date data it is better to use custom fields, as the data is more variable, and is less reusable.
Either way, using custom fields for everything is still perfectly fine, as long as you are not planning to have a huge amount of posts (like 100 000+). In those cases it is very important to use the most efficient data structures, but for regular uses it is fine to have the most convenient setup for yourself or the client.
I hope this helps!