These researchers are the same ones who developed FB23 and...

These researchers are the same ones who developed FB23 and FB23-2, and it seems they're still trying to optimize it as they can see the potential of FTO inhibition. They wouldn't use bisantrene when they're trying to develop their own inhibitor.

“Because of the poor permeability of FB23 in cells, further optimization based on the bioisosterism principle was applied to modify the carboxylic acid of FB23, which led to the discovery of the benzohydroxamic acid FB23-2, which has an IC50 value of 2.6 μM.” - Targeting the RNA demethylase FTO for cancer therapy

From Su et al (which they even cite):
"More recently, two derivatives of MA, termed FB23 and FB23-2, have been developed, which showed improved efficacy in inhibiting FTO activity and viability of human acute myeloid leukemia (AML) cells. Nonetheless, their IC50 values in inhibiting AML cell viability are still >1 mM (FB23-2) or even >20 mM (FB23) (Huang et al., 2019). While FB23-2 showed a statistically significant effect on inhibiting the progression of human primary AML in mice, which provides proof-of-concept evidence indicating the therapeutic potential of pharmacological targeting FTO in treating AML, theinhibitory degree was not satisfactory. Thus, there is still an urgent and unmet need to develop efficacious inhibitors against FTO to treat AML and other cancers."

They're not pretending bisantrene doesn't exist like others, are aware of the shortcomings of FB23/FB23-2, yet still seem intent on creating a usable drug. My question would be - how far away are they from their nearest competitor? Bisantrene still has a strong lead.