SPF works on the envelope sender - ie
MAIL FROM: during the SMTP conversation, rather than the 'From:' in the body of the email.
Often these are the same. So the domain specified in 'MAIL FROM:' is somedomain.com first. The SPF process on the server accepting example.com email would look up the address of the incoming SMTP server and compare it against the somedomain.com SPF TXT DNS record.
But then you say it is forwarded to another address at example.com - this does not require an SPF lookup, as it is a local delivery - no other SMTP server participates.