David Hobson got his information from the research of Gary Giuffré, whom he now persecutes. So, the research regarding Cardinal Siri and the 1958 conclave should be able to be discussed apart from DH.
As to why an underground hierarchy would stay underground in the present circuмstances, I will just say that it is easy to make judgements for those who are not in that particular situation themselves. The latter cannot know what exactly the motivation or reason could be why they still have not made a public resistance against the masonically controlled society.
It is in any case known, and captured on video tape, that the Italian Abp. Pintonello (before a group of traditional Catholics) explicitly confirmed that Cardinal Siri was the Pope, and that he made a cryptic allusion towards his succession.
The Marquis de la Franquerie, who was a friend of Marie-Julie de Jahenny (the Breton stigmatist) and Secret Chamberlain of His Holiness Pius XII, is also said to have believed that it is true, after the famous conversation which he, along with two other persons, had with Card. Siri.
These were two reputed men in the Church, so I would personally not disregard these testimonies too rashly.