When sharing the url with twitter validation, it should be done using the custom domain. You have shared the webflow.io domain and when that opens you can see the root domain added to the sitemap.xml:
The root domain can only be set as the default domain if you are using a CNAME/ANAME record on the root domain to point to proxy-ssl.webflow.com.
If you are using DNS A Records to point the root domain at Webflow IP addresses instead, then the WWW domain need to be set as the default domain in Webflow, see how to set the default domain: Set a default domain | Webflow University
Both domains need to be added to the hosting tab like:
www.example.com (with the WWW domain set as default if the A Records method in DNS is being used).
After making changes, republish the site and then the share should work correctly. It sounds like the A Records are being used, the root domain is set as the default domain, or the WWW domain has not yet been added to the custom domain tab of project settings and set as the default domain.
Side note, the webflow.io domain has robots.txt instruction to deny site indexing by default, if you actually wish to index the webflow.io domain, you can do so by disabling the setting to prevent indexing on the seo tab of general settings:
I hope this helps