No part of the D.C. Sexual Abuse Statute of Limitations Amendment Act of 2018 applies in the case of Tara Reade. The statue of limitations for criminal prosecution had already expired under the existing law and was not revived by the amendment.
You are correct that there is a difference between the statute of limitations for criminal prosecution and civil suits. Phoog's answer cites information that is specifically about civil suits, and as it says, the statue of limitations for civil action has also expired. (I have not seen any reports of Reade filing a civil suit.)
Tara Reade filed a criminal complaint with the Washington Metropolitan Police Department
Reade also confirmed that the statute of limitations around the claims against Biden have passed.
"I filed a police report for safety reasons only. All crim [sic] stats beyond limitations. Gratitude for all who have stood by me," Reade tweeted.
("JOE BIDEN SEXUAL ASSAULT ACCUSER TARA READE FILES CRIMINAL COMPLAINT", by Ewan Palmer, 4/11/20, newsweek.com)
The web page Sexual Abuse Statute of Limitations Amendment Act of 2018 - Two Year Window Guide, on http://www.davidgrosso.org/, has a link to fact sheet summarizing the changes.
There is no statute of limitations for criminal cases regarding sexual abuse committed after May 3, 2019. But sexual abuse crimes committed earlier whose statute of limitations had already expired by that date under the old law cannot be prosecuted.
Section 4(a) shall apply to an offense committed before, on, or, after the effective date of this act, unless the statute of limitations for the offense expired before the effective date of this act.
D.C. Law 22-311. Sexual Abuse Statute of Limitations Amendment Act of 2018., Sec. 5. (b)
The previous statute of limitations seems to have been 15 or in some cases 10 years, so for a crime committed in 1993, the statue of limitations would have expired by 2019.
The 2-year revival period only applies to civil suits:
Notwithstanding any other provision of law, a claim for the recovery of damages that would be time-barred under D.C. Official Code § 12-301 before the effective date of this act, but that would not be time-barred under section 3, is revived and, in that case, a cause of action may be commenced within 2 years after the effective date of this act.
(Sec. 5. (a)(2))