What can the world do? Social distance. Masks. Contact trace. Etc.
The world hasnt had a slow and ineffective response against covid19. Some countries have have been extremely poor in their response. You know who they are. Some countries or states have been extremely proactive and their citizens have remained alive and well.
As an example my state: "South Australia". Population 1.6 million. 4 deaths. All known cases in hotel quarintine, 30 days since last known community spread. When the virus got out in the community last in mid November, we had what I believe was the toughest snap lockdown on the planet (couldn't leave home even for even exercise or work), with 12 hours notice, and masks were made compulsory in many places. That was kept up for 3 days before our contact tracers got on top of the problem (they requested 6 days but it ended earlier as the virus was less contagious than wed thought). We put 4000 people in quarantine and the cautiously reopened. Now life is back to covid-normal. Some big sporting events and some cross border travel but still staying 1.5m apart.
We have many of the problems other places did. Big open land borders with neighbouring states. Towns on the borders that couldnt easily be segmented. An international airport. Densely packed public transit and people packed like sardines in lifts and nightclubs.
The secret? A boring, forgettable, leader who listens to health advice and a population who may grumble but follow instructions for the good of their neighbour.
But what happened to your cults plan?
Your cults plan will definitely have another pass over the "I'm protesting cause I need a haircut" crowd and anyone living in a city with more than a few of them. Id expect several red American states to be wiped out. You may destroy several countries even: India, Brazil, UK, parts of Africa. Maybe the EU.
But those with border quarintine, contact tracers, mask wearers, lots of hand washing, and social distancing, will just sigh and continue life as normal.
Eventually your cult will accidently infect themselves in their lab and spread the disease within their bunker, killing themselves off.
The viruses they do release will mutate over time to become less deadly (they spread further if they dont kill host). A year after they wipe themselves out their super covid will be no more deadly than the current one. Existing vaccine research could pivot to include them (AstraZeneca stated it would take 6 weeks to create a new vaccine for a variant of covid).
2023 Will be marked by Australia, NZ, Oceania, Southeast Asia, China, and a few enclaves in the USA and EU dropping their borders, emerging from lockdown, and regretting calling 2020 the "worst year ever".