In this scripture the woman is (primarily) praising not Mary, but Jesus.
It is common to express praise for someone indirectly like that. For example, today, you might tell someone: “Your husband is a very lucky and blessed man” meaning to say that the husband is very lucky and blessed to have such a wonderful wife. The praise is directed not to her husband, but rather to her. Likewise, here the praise is directed not to Mary, but to Jesus.
So Jesus is addressing this woman’s praise of Jesus, not any praise she had for Mary. It may be that this woman was praising Jesus because he was viewed as a great or wise teacher. Jesus is explaining to the woman that it is not being a great teacher that is blessed, but rather that it is hearing the word of God and keeping it – something that Jesus did and exemplified – that is blessed.
In St Bonaventure’s Marian psalter you see another example of the same kind of language: “
Blessed is the
womb that bore thee: and
blessed are the
breasts that
nourished thee,” where it is clear that it is not Mary’s mother that is being (primarily) praised here, but rather Mary herself.
So the objection based on this scripture is premised on a misunderstanding of what the woman was doing and what Jesus was responding to.
I’ve been told that in this scripture Jesus was trying to emphasize that His kingdom was based on new spiritual relationships rather than on familial relationships that formed the basis of Israel. But I don’t know if I’ve been told right.
Besides that, it may be that Jesus was trying to protect His mother. If He had proclaimed or advertised her as the Mother of God, Immaculate Conception, one who would crush the head of the serpent, etc., then he may have caused her to suffer unnecessary danger or persecution. It’s just like when he protected himself from unnecessary danger and persecution when He instructed his disciples to not tell anyone that he was the Christ. He needed to first complete His public ministry before traveling the
Via Dolorosa. When His public ministry was completed in accordance with the Father’s will, He did reveal Mary to us as our Mother, saying, “Behold, your mother.”