Good lord what a horrible recommendation. This is like telling someone to learn programming by starting with assembly.
If you want to actually learn subtractive synthesis minus the complexity use an all in one synth VST like Surge which is free and open source and you won't have to worry about tedious fundamentals that don't actually matter unless you're doing modular synthesis. Helm is another great VST.
Once you understand subtractive you can graduate to more complicated methods of synthesis like FM, vector, ETC.
If you want to actually learn subtractive synthesis minus the complexity use an all in one synth VST like Surge which is free and open source and you won't have to worry about tedious fundamentals that don't actually matter unless you're doing modular synthesis. Helm is another great VST.
Once you understand subtractive you can graduate to more complicated methods of synthesis like FM, vector, ETC.