Your assumption is correct. The answer is No. The delinquent withdrawal satisfied your 2018 RMD requirement. You cannot use the same withdrawal to also satisfy your 2019 RMD requirement.
Note that all withdrawals are reported as income in the year that the withdrawal is made, so your RMDs for both 2018 and 2019 will be reported as income on your 2019 tax return. The Form 1099-R that you get from the custodian for 2019 will include all the withdrawals that you made in 2019, even though one of the withdrawals was to satisfy your 2018 RMD requirement.