I checked 2017 all time high:
- Bitfinex: $19,891
- BitMEX: $20,093 (not $20,086)
As far as I know, CMC shows a price which is the average of more exchanges... As a consequence, if BTC was 20.093$ at BitMex on 17.12.2017, if this value was added with other values then CMC calculated an average, it could lead to the amount shown of 20.089$.
However, I certainly remember that day that I read that BTC surpassed 20.000$ (can't find the article though).
And, if CMC is not wrong (it could be, but chances are low), then on 01.12.2020 BTC got close to the ATH, without reaching a new ATH.