The FRT decision has nothing to do with the idea of reducing congestion. It has everything to do with lost revenue. Effectively for every 700k Tesla, they were forgoing ~700k of tax revenue. If the average selling price of an EV eventually is around 350k (pre-tax), that would be ~300k in lost revenue per vehicle. I am not happy about the decision, but lets face it - it was too good to be true that the effective subsidy was BY far the highest in the world - we are talking about US$80-90k for a Model S.
HK has annual auto sales of around 50k new passenger vehicles per year. If one day 30% of those sales are electric and the government did not collect on average say 200k in FRT on those vehicles, that costs the government US$385m a year. If you ask me, its a price worth paying - but in the end its politics.