That's absolutely ridiculous, their costs are nowhere near that. Their rideshare price is $5500/kg, and every kg they sell is displacing their own payloads so there's simply no way they're pricing it at 7% of their costs. Even excluding R&D Shuttle was far more costly.
All rocket ridesharing is really cheap, because it's essentially free once the rocket has been paid for by the main payload. The only real cost to the rocket provider is ensuring that you don't interfere with the main payload.
That would be true if the main payload left the rocket with spare capacity but that just isn't the case. SpaceX's main payload can use every last kilogram of capacity. Every kilogram they sell as rideshare is a kilogram they lose for Starlink. They'll still have to pay the cost to launch that kilogram later so there's nothing free here. As I already pointed out...
There are two types of Starlink rideshare. Transporter goes to SSO and is dedicated to rideshare. But the other is Starlink adjacent LEO which bumps a Starlink or 3 for your payload. There haven't been many of those but there have been a few.
