I don't know anything about Horus Heresy or Warhammer 40,000, but since you don't know of any relevance of Loginus in existing Warhammer 40K lore, maybe the name is supposed to be significant outside of Horus Heresy and Warhammer 40K.
There was an ancient Roman gens or clan Cassia, one branch of it was the Cassius Longinus family with many known members from Quintus Cassius Longinus, mentioned in 252 BC, to Gaius Cassius Longinus, consul in AD 30, alive in the time of Vespasian.
One Gaius cassius Longinus was a leading instigator of the plot to assassinate Caesar in 44 BC.
There were other Roman families with the family name Longinus, belonging to other clans, for example there was a Roman general Gnaeus Pompeius Longinus who died in AD 105.
Lucius Cassis Longinus was Consul in AD 30, and was married to Juliu Drusilla, sister of Caligula and great granddaughter of Augustus, from 33 to 37. Junia Lepidia, a great great granddaughter of Augustus, married a Gaius Cassius Longinus and was the grandmother of a Domitia Longina who married Emperor Domitian.
In the Gospel of John (19:31-19:37), usually dated to about AD 80 to 100, a Roman soldier is said to have speared the body of Christ on the cross to see if he was dead. The spear used by that soldier is called "The Holy Lance", "The Spear of Destiny", etc., and several alleged holy lances have been venerated as it in various places.
A text called the Acts of Pilate or Gospel of Nicodemus is an apocryphal gospel possibly written in the fourth century (AD 301-400).
The Gospel of Nicodemus names several minor New Testament figures who were not named in the canonical texts; for example, the soldier who speared Jesus on the cross is named as Longinus and the two criminals crucified beside Jesus are named as Dimas and Gestas.
The name is probably Latinized from the Greek lonche (λόγχη), the word used for the lance mentioned in John 19:34.
So not only were there famous historical people named Longinus, a character in the New Testament was later named Longinus, and as you might expect, there were many strange stories written about that Longinus by various Christian writers between late antiquity and the present.
Longinus did not start out as a saint. An early tradition, found in the 4th-century pseudepigraphal "Letter of Herod to Pilate", claims that Longinus suffered for having pierced Jesus, and that he was condemned to a cave where every night a lion came and mauled him until dawn, after which his body healed back to normal, in a pattern that would repeat till the end of time.
Longinus is venerated, generally as a martyr, in the Roman Catholic Church, the Eastern Orthodox Church, and the Armenian Apostolic Church. His feast day is kept on 16 October in the Roman Martyrology, which mentions him, without any indication of martyrdom, in the following terms: "At Jerusalem, commemoration of Saint Longinus, who is venerated as the soldier opening the side of the crucified Lord with a lance". The pre-1969 feast day in the Roman Rite is 15 March. The Eastern Orthodox Church commemorates him on 16 October. In the Armenian Apostolic Church, his feast is commemorated on 22 October.
The statue of Saint Longinus, sculpted by Gian Lorenzo Bernini, is one of four in the niches beneath the dome of Saint Peter's Basilica, Vatican City. A spearpoint fragment said to be from the Holy Lance is also conserved in the Basilica.
Longinus and his legend are the subject of the Moriones Festival held during Holy Week on the island of Marinduque, the Philippines.
More than one modern writer has depicted Longinus as destined to walk the Earth until the second coming of Christ, much like the Wandering Jew of Medieval legends.
For example, the television program Roar (1997) had a Gaius Cassius Longinus among the Roman invaders of Ireland in AD 400.
One of the major villains in the program was Longinus, played by Sebastian Roché, an immortal cursed by God for interfering with his plans. By Christian tradition, Longinus was the centurion who stabbed Jesus Christ with his spear during the Crucifixion. This spear, the Spear of Destiny, was supposedly the only weapon that could release Longinus from his curse. The show freely mixed Christian mythology, Celtic mythology, Druidism, and smatterings of history.2
Therefore, it is possible that the name of Longinus is supposed to evoke memories of Gaius Cassius Longinus who led the plot to assassinate Gaius Julius Caesar, or of the Longinus of Christian tradition, who has been considered both anti Christian and as a Christian convert, saint, and martyr, and is sometimes depicted as immortal.