The data shows there was less than one inter-caste marriage per caste pair per generation for the last few thousand years. What was left to become "more rigid" than that under the British?

For one thing, there is often a subcaste structure within each local caste itself. The subcastes of a caste who received British patronage, i.e. jobs in the colonial administration, would preferably marry only among themselves.

Of course, this was about preservation of wealth and power, and not so different from similar marriage practices among wealthy classes in England (as depicted in Downton Abbey)

The British didn't invent the sub-castes, but they effectively exploited them.