Absolutely false. Some MD5 collision-generators specifically find pairs of equally-lengthed inputs with the same hash. See for example hit #2 for [MD5 collisions]:
'Extension attacks' are something else, which let you turn one collision into more, or create valid hashes for combinations of unknown text plus a chosen extension – not find an initial collision. See:
The 'length extension' property can be helpful, once you find a collision based on 'random' nonsense, in extending that into two documents that are each meaningful-but-different and still colliding, as was done in this 2005 MD5 collision demonstration: