Aquinas is saying, if I interpret him correctly, that it is possible for us to harden our hearts against the Holy Spirit. I’m sure people have done this (there are examples in Scripture) and still do it. A good reminder to always keep our hearts soft towards God–his corrections of us and his will for us.
I do think he is indicating something more than hardening of heart which can come about through ignorance, weakness and carelessness. He is speaking of what classifies a sin unforgiveable as having a ‘certain malice’ or choice motivating it. To the objection that this quality is determined at the finality of a life he answers that that quality can be present prior to sin being committed thus rendering the person gravely condemned at the moment of the sin. When you look at Mark 3 where Jesus has spoken the words about the unforgiveable sin regarding the Pharisees, at the beginning of the chapter we can see that ‘certain malice’ already established when they witnessed Jesus healing power and capacity to drive out demons. “Then the Pharisees went out and began to plot with the Herodians how they might kill Jesus.” - Mark 3:6
Aquinas in the article prior says…
"According to the various interpretations of the sin against the Holy Ghost, there are various ways in which it may be said that it cannot be forgiven. For if by the sin against the Holy Ghost we understand final impenitence, it is said to be unpardonable, since in no way is it pardoned: because the mortal sin wherein a man perseveres until death will not be forgiven in the life to come, since it was not remitted by repentance in this life.
According to the other two interpretations, it is said to be unpardonable, not as though it is nowise forgiven, but because, considered in itself, it deserves not to be pardoned: and this in two ways. First, as regards the punishment, since he that sins through ignorance or weakness, deserves less punishment, whereas he that sins through certain malice, can offer no excuse in alleviation of his punishment. Likewise those who blasphemed against the Son of Man before His Godhead was revealed, could have some excuse, on account of the weakness of the flesh which they perceived in Him, and hence, they deserved less punishment; whereas those who blasphemed against His very Godhead, by ascribing to the devil the works of the Holy Ghost, had no excuse in diminution of their punishment.
…Secondly, this may be understood to refer to the guilt: thus a disease is said to be incurable in respect of the nature of the disease, which removes whatever might be a means of cure, as when it takes away the power of nature, or causes loathing for food and medicine, although God is able to cure such a disease. So too, the sin against the Holy Ghost is said to be unpardonable, by reason of its nature, in so far as it removes those things which are a means towards the pardon of sins. This does not, however, close the way of forgiveness and healing to an all-powerful and merciful God, Who, sometimes, by a miracle, so to speak, restores spiritual health to such men."
So the unforgiveableness of the sin can be believed to be referring to its nature also rather than only the state of sin at the moment of death.
That lesson could be warning us that like the Pharisees, so knowledgeable about the ways and promises of God, but could be overcome by envy of Jesus spiritual good and non-conformity to their ways, that they reject the Holy Spirit in Him… we also have the capacity to reject the Holy Spirit in our journey to heaven motivated by certain malice.
But, we cannot know who is truly reprobate and who isn’t, nor can we judge anyone based on our perceptions. God knows the heart better than anyone else and all the mitigating circumstances that speak to one’s culpability. We must carefully guard our hearts against malice so we don’t fall into this sin.