Here is the Silabs explainer on the certification process: https://docs.silabs.com/matter/latest/matter-certification/
You can! You just can't ship it/sell it without certification.
https://www.reddit.com/r/homeassistant/comments/1adh8ah/esp3...
I did exactly that last week... Certification is required if you want to use the trademarked logo in your marketing materials (same as with Wi-Fi and Bluetooth afaik).
Here is the Silabs explainer on the certification process: https://docs.silabs.com/matter/latest/matter-certification/