Re: Since when
The MP3 compression algorithm is just a mathematical formula but is patented and a licence is needed to encode audio to MP3. The argument is that a lot of work went into inital studies and testing to find a suitable set of weighting factors to give a balance between compression ratio, ease of operation and fidelity. The MP3 encoding technique (and just about all codecs) is based on decades old mathematical research that is free for everyone to use (the formulae), but you'd better not copy the mix of methods and the internal weighting factors of a well known one, or you will be sued.
It sounds like the Twinkle Twins have copied the basic technique (time varying, multi dimensional weighted averages, whatever...) and I'm sure they did a lot of original work and much testing before they hit upon their own unique weighting parameters.