If a so-called Valid Priest gives a Valid Mass. Perhaps under certain circuмstances only If I am sure if the if the priests were valid.
I knew an old frail Catholic Priest who was ordained before the changes, who is your regular old diocesan priest and he has a TLM. He showed me his Ordination Card and Pictures and he seemed to have not cared about the changes and just celebrated the TLM like nothing changed! He was quite sound and traditionalist in his teaching so I did go to this Mass when I was in the area.
Yes, there used to be a similar situation in my area. But that priest died a few years ago. Not too many still alive who were ordained before 1970.
The issue with the indult is that there is a triple-doubt:
1. Doubt associated with the consecrating bishop/consecration formula. (Not a lot of bishops alive who were consecrated prior to 1970.)
2. Doubt associated with the ordination formula.
3. Doubt associated with the new rite (or some hybrid old/new rite).
There is positive doubt and canon law does NOT ALLOW attendance at positively doubtful sacraments/masses, without grave sin (except of the situation is an emergency).