The thinking at the time was that the teflon would help the sabot to come off better. The hardest part of DS design is getting rid of the sabot after the package leaves the barrel. That is why DS small arms flechettes have not worked so well. Designing a sabot which would hold onto the flechette well enought for handling,loading and firing but get lost asap after leaving the barrel to avoid affecting accuracy is a MAJOR task. Thousands of experiments were done to develope such a sabot and NONE worked at acceptable parameters. The larger the caliber the easier it is to get rid of the sabot once fired BUT it has to fall off without degrading the trajectory of the projectile. Projectiles without tails work better with sabots of most designs. The Teflon wasn’t necessary but there was a “coat everything with Teflon” push for quite some time. You will find US small arms cartridges entirely coated with Teflon which were used in Arctic tests to keep the ammo from sticking in the chamber. teflon is not a rust preventative. It is quite porous.
Take a look at the complecated design of the sabot today on you DS rounds. These designs are the results of thousands of design experiments mostly dealing with shedding the sabot once it hits the air stream. You don’t see any Teflon on the TERMINATOR - that is GOLD. OH YES!