They have to wait so many weeks until after the decree nisi is granted to apply for the decree absolute which formally ends the marriage so yes atm they are still legally married.
Edit - 6 weeks and 1 day
https://www.gov.uk/divorce/apply-for-a-decree-absolute
The decree absolute is the legal document that ends your marriage.
You need to wait at least 43 days (6 weeks and 1 day) after the date of the decree nisi before you can apply for a decree absolute.
Apply within 12 months of getting the decree nisi - otherwise you will have to explain the delay to the court.
In this case, the decree nisi was granted on February 17, 2017 (nearly two years ago), so the waiting period for both of the spouses has elapsed. (From the above link, the petitioner is allowed to apply for decree absolute following the 6 week and 1 day waiting period, but the respondent must wait an extra three months.) Nevertheless, the
judgment does make it apparent that there has been no decree absolute and Prince Louis and Princess Tessy are legally married.
The judgment points out the dates of the marriage breakdown, the application for financial remedy orders, and the grant of decree nisi, but there is no reference to any decree absolute.
Very sadly, the marriage broke down in the summer of 2016. The husband moved to Paris where he currently resides with his sister in a family property. The husband expressed an intention to the court to move out of this property and into rented accommodation funded by himself as soon as he is able to afford to do so. The children spend time with each parent but at present reside with the wife in the former matrimonial home in London. Decree nisi was granted on 17 February 2017. Application for financial remedy orders made on 25 January 2017.
The judge noted he was asked to decide about the fate of Princess Tessy's titles following the granting of decree absolute. That, too, suggests the decree absolute is in the future.
I determined that this court did not have jurisdiction to decide issues concerning the fate of the wife's Royal titles following the granting of decree absolute, the arrangements for communication between the wife and the Royal Family of Luxembourg following the conclusion of these proceedings or the compilation of a joint Press Release from the wife and the husband concerning these proceedings.
As the judge was also asked to decide "issues concerning … the compilation of a joint Press Release from the wife and the husband concerning these proceedings", I think we can expect a press release from at least one of the spouses if and when the couple finally divorce.