Your reliving is already approved and it is sufficient to join the new company. Everyone knows the situation that due to lockdown people cannot come to office and hence cannot return company property. Only your full and final is withheld, subject to you returning the assets and completing KT.
KT period : The best course is to negotiate this with your manager ( the one who manages the old team where you need to give handover) and understand what is needed. This should have been done by you in your notice period. If you have done the same, document that in an email and ask your manager to acknowledge that KT is complete.
If KT is not done, prepare a KT plan and find out its duration. Are there other people in the team with similar skill set? Do you possess any knowledge which is unique and is not known to other team members? Do you have documents to cover what needs to be done in your role ( e.g. how to do a certain task like build etc). If not, can you prepare some documents?
Due to COVID-19 situation, employees are locked in and cannot go to office to return the hardware. In such instances, F&F is withheld to ensure that the property is returned safely. Your company HR has gone one step ahead and included a KT disclaimer.
In most cases, handover has to be negotiated with the manager rather than HR. If you already have trained a back up, then there should be no concern as you don't need a KT period and manager can waive it off. If not, you can follow step 3, create documentation to help someone play your role. You can ask your manager for the new person to go through the documentation and redirect any questions to you, which you can address in person for one day when you come to office for your F&F.
It is best to negotiate something which suits both parties rather than get into a fight. If its 2-3 days, I would recommend you take a few days off from your new office and get it done. If you will explain it to HR/manager in your new company, they will understand as they would have a few similar cases and accommodate you. If the old company asks for something extensive, then a lot will depend on your relationship and negotiation skills.
You can put your foot down if they ask for an extended KT, however, the laws are pretty grey and wont help you. Worse case scenario would be to ask them to document the extended KT duration in writing and approach a lawyer. Most organizations will back off, unless someone wants to make an example out of you.