Why can "yea' and "nay" be used interchangeably here?

The definitions of Yea and Nay, provided by Lexico, are:

Yea;

Used to emphasize a more appropriate word than one just used.

‘he was full, yea, crammed with anxieties’

and Nay

Or rather (used to emphasize a more appropriate word than one just used)

‘permission to build the superstore will take months, nay years’

What, if any, are the historical reasons as to why ‘Yea’ and ‘Nay’ are semantically identical with regard to the context of introducing ‘a more appropriate word than the one just used’?

Did their meanings arise independently of one another?

Is my interpretation that they’re semantically identical, incorrect? Do they in fact have slightly different connotations when used in this context?