A Proxy Changes the Website-Facing IP, Not a MAC Address in HTTP
Distinguish local-link hardware addressing from the source IP seen by a remote website through a proxy.
A network adapter's MAC address belongs to local-link communication. It is not an ordinary field in an HTTP request that travels unchanged across the internet.
When an application uses a forward proxy, the proxy opens the destination-facing connection. The website observes that connection's source IP. It does not receive your original Ethernet frame merely because the proxy relayed the HTTP exchange.
On your local network, the operating system still selects an interface and the link-layer addressing needed to reach the next hop. That is a different scope from what the website sees.
curl's interface option can influence which local interface or source is used where the platform supports it. It does not turn a MAC address into an HTTP source identity or guarantee a different public exit address.
An application can explicitly send device identifiers as data. Such a value would be application content, separate from the normal behavior of IP routing and proxying. Changing the proxy does not erase information the application deliberately includes.