No it is not true.
If a person is working ands has another insurance Medicare is secondary.
If you stop working and have another insurer to cover you Medicare will remain secondary.
None of this is automatic but must be done by contacting ssi.
-PG 102 Lines 12-18 HC Bill - Medicaid Eligible Indiv. will b automat.enrolled in Medicaid. No choice
http://www.economicpolicyjournal.com/2009/07/whats-in-healthacre-bill.html
Not sure if this applies to Medicare, also, but my gues is yes. The devil is in the details.