The only difference between a microchip & a signed photo ID/"proof of origin" paperwork is that the microchip can't become separated from the snake. Neither one is going to make a het out of a normal if someone is really hellbent on ripping people off & using either method to do so.
Whether or not the snake is guaranteed legitimately still comes down to acquiring animals from a reputable source that you trust & with whom you feel comfortable doing business. If part of that means having microchip ID available, then buy chipped snakes.
Just my $8.02 (includes fuel surcharge).
K~