No, it's not a myth.
The Pilgrim's Office website states
You must collect the stamps on the “Credencial del Peregrino” from the places you pass through to certify that you have been there. Stamps from churches, hostels, monasteries, cathedrals and all places related to the Way are preferred, but if not they can also be stamped in other institutions: town halls, cafés, etc. You have to stamp the Credencial twice a day at least on the last 100 km (for pilgrims on foot or on horseback) or on the last 200 km (for cyclists pilgrims).
It doesn't mention special dispensation for those who have walked farther.
Sure, many pilgrims who started from SJPDP or some other point several hundred km from Santiago have received a Compostela without two stamps per day during the last 100km, but it's at the discretion of the volunteer or staff member when you arrive at the Pilgrims Office.
Please make it easier on those volunteers and staff members and follow the rules.
The “Compostela“, the accreditation of the pilgrimage to the Tomb of St. James.
oficinadelperegrino.com