The early definition of free software (set forth in the GNU’s Bullentin Volume 1, Number 1 in 1986) was unknown to the authors of the Debian Free Software Guidelines in 1997. This early definition was much weaker than the DFSG and it seems that The Free Software Definition had not yet been published as such.
In fact, the 1986 GNU’s Bulletin definition was not the modern “Four Freedoms”, but a simplified version that focuses on the abilities to redistribute and change programs (but not specifically the ability to redistribute changed programs!). This early definition is close to the “modern” freedoms two and one.
The DFSG were first published in the July 1997 announcement of the Debian “Social Contract”. It explicitly mentions the ability to redistribute modified source code (or at least “original plus patches”). This was not explicit in the early GNU’s Bulletin definition, though it is related to “modern” freedom three.