If I understand your situation correctly, the Munchkin code should update the corresponding Marketo lead record with the Vist Webpage activity and the URL of the webpage. However, the lead must. have clicked a link in a Marketo email or filled out a form on the landing page in order for it to update the corresponding Marketo lead record; otherwise it will create an unknown lead record.
If the number of visitors is small, you may be able to manually match the unknown and known records after the fact using a smart list with a Visited Web Page filter and the Anonymous IP filter wtih the IP address of the unknown lead that visited the web page, you could then merge these records together. Alternately, you could synch the personalized URL to the Marketo lead records and use that field as a filter with the Visited Web Page filter ("use ANY filters" option).