Half information isn't really facts. Issue isn't about the meet, which happened before ban was announced, issues is that at least 1,500 people continued to stay put even when the meet was over.
This is the timeline and facts
March 13: 3400 people gather at the Nizamuddin markaz as part of a religious gathering.
March 16: Delhi CM Arvind Kejriwal announces that no religious, social, political gatherings of more than 50 people are allowed in Delhi till March 31.
People at the Nizamuddin Markaz still continue to stay put.
March 20: 10 Indonesians who attended the gathering in Delhi test positive in Telangana.
March 22: Prime Minister Narendra Modi’s Janta Curfew is observed across nation. No public gatherings were allowed for a day.
March 23: 1500 people vacate the Markaz.
March 24: PM Modi announces nationwide lockdown for 21 days. No public gatherings, any kind of non-essential movement outside residences allowed. Only essential services allowed to remain functional.
March 24: Nizamuddin Police asks remaining people at the Markaz to vacate the area.
March 25: Around 1000 people are still residing in the area defying lockdown orders. A medical team visits the Markaz and suspected cases are isolated in a hall within the building. Jamaat officials go to the SDM's office. File an application for permission to vacate. Vehicles list was also given to seek passes.
March 26: An Indian preacher who attended the gathering in Delhi is tested positive and dies in Srinagar.
March 26: SDM visits the Markaz and calls the Jamaat officials for a meeting with the District Magistrate.
March 27: Six coronavirus suspects are taken away from the Markaz for medical checkup and are later put in a quarantine facility in Jhajjar, Haryana.
March 28: A World Health Organisation (WHO) team along with the SDM visit the Markaz. 33 people who were taken for medical checkup are isolated at Rajiv Gandhi Cancer Hospital in Delhi.
March 28: ACP, Lajpat Nagar, sends a notice to the Markaz to vacate immediately.
March 29: Markaz officials respond to the ACP's letter saying no new people were allowed to gather post the nationwide lockdown announcement. The current gathering had started much before the lockdown and that the PM in his lockdown speech said jo jahaan hai, vahin rahe (stay put wherever you are).
29 March night: Police and health authorities start taking out people from the Markaz and send them to hospitals and quarantine facilities.
So, while the meet and its continuation was stupid, reckless and illegal, it is not really an excuse to target an entire religion as if everyone is guilty and all this was done with malice in mind.