"as ... as any ..." is old-fashioned and literary, it reminds me of folk tales and fairy stories. I have heard:
"as green as any grass"
"as green as any glass"
"as gay as any gypsy caravan"!
Your first or third interpretations are both correct. It could mean "as modest as an ordinary matron" or "as modest as the most modest matron". In the way this expression is used, the difference is not important.