Buy back isn’t unlimited… they are long term investors, wouldn’t make sense to sell e.g. MFT to raise cash just to buyback shares.
They should only buyback enough to hold for treasury stock to give out in the dividend reinvestment scheme.
Otherwise they should be buying using what cash they have while everything is so cheap.
I recall reading that the fund can borrow up to 20% (by memory) to invest. Never seen that done before but that would be an interesting move