SomeKindOfFloppy
The difference with Brits in EU countries is that they were legally entitled to stay long term or forever, with no need for formal residency or a work visa at the time they went there. The goalposts were been moved on them due to Brexit. They didn't go to France or Spain in the full knowledge that they had no right to live and work long term.
Even if these Indian chaps arrived completely legally (student visas for example) they at some point made the conscious decision to outstay whatever visas they had. They have not had the goalposts moved on them, like Brits in the EU have.
There have been many Brits living 'under the radar' in France and Spain full time without completing tax returns or paying any tax, without registering their UK cars and driving them all year round on UK insurance etc., as indeed there have been EU citizens who have been doing exactly the same in the UK.
Brits were not were not obliged to formally apply for residency if they wanted to stay for more than 183 days in a year, nor to fully register themselves into the French healthcare system with private mutuelle top up insurance, but they should have completed a tax return if they stayed over 183 days in the year, which of course many did habitually did. But but with no-one monitoring their travel movements within the EU it was very easy to stay much longer.
There is a double taxation treaty between France and the UK so even if they had completed a tax return they would not have been taxed twice on a UK based income such as a pension, or investments or remote working anyway.
The slight difference is that many of the Brits in Spain and France who have gone 'under the radar' are often retirees, entirely self funded and not looking for work, either on the black or by entering into a job contract obtained by giving false information about their right to work. That's not to say that there haven't been British people working in the EU and avoiding tax - of course there have. But that was their crime - avoiding tax. Not working illegally or being an illegal immigrant.
As for the link to that article, about British people 'starting their illegal stay' it's a bit misleading.
UK citizens wanting to live in the EU have up until 30th June 2021 to apply for residency, providing they can show that they started the process of settling (eg having a tenancy agreement in place) before the end of the transition period.
So no, many of them will not have 'started their illegal stay' from 1st April. They are still within their rights to be there . Even once 1st July comes and goes, they still won't be illegal because they have until October to complete the residency process.
No-one who has a house there, either owned or rented, can be shown to be an illegal over-stayer until after 1st July, if they have not submitted an application for residency by then. So the illegal part of their stay will begin then.