I strongly doubt Jenny was better than Sifan till 2019, their career doesn't say this.
1) There are 7 years of difference : Simpson was born in 1986, Sifan in 1993.
2) Sifan looked at short events till 2017, with average in her 10 top 800m (from 2014 till 2018) of 1'58"948, better than the PB of Jenny (2'00"45 in 2013)
3) Between 2014 and 2019, Sifan had an average (top 10 performances) in 1500m of 3'55"99, also in this case better than the PB of Jenny (3'57"22 in 2014)
4) Already in 2014 the PB in 3000m of Sifan was better than the PB of Jenny (8'29"38 vs 8'29"58)
5) Always Sifan was better in longer distances.
6) Jenny was specialized in 1500m after some year running steeples. She won WCh in 2011, took silver in WCh 2013 and 2017, and bronze in OG 2016.
Sifan won WCh indoor in 2016 and bronze in WCh 2015-2017, before the unusual coupling 1500/10000 producing two World titles in 2019.
7) The range of Sifan always was very much wide. She started when only 18, already running 15 km in 53'57" and a HM in 1h17', and at the moment has the WR of one hour, and a PB of 65'15" in HM (in my opinion she can better the WR after Olympics. She was able to better the WR of Mile and there is no possibility of compare the career of the two athletes, also without considering what Sifan was able to achieve after 2018.