Of all the vessels in the service of the Adeptus Astartes, the Nova class
frigate is commonly the single class to which the Imperial navy take the
most exception. It lacks sufficient size to really fulfil the deployment and
assault roles for which Space Marine ships are primarily intended and its
lance armament and speed make it a menacing gunboat in its own right. As
such, the Nova remains rare in most Space Marine fleets, a trend the
Imperial Navy, the Inquisition and other institutions perpetually concerned
by the balance of power would dearly love to see continue.
Bold Part:
Lances and the position of the Imperium & Inquistion regarding them. Pretty clear.
No, the word that should be highlighted is 'gunboat' or 'gunship'. It's mentioned again and again in the fluff about things SM can't have bigger then escorts. Lances are only mentioned once in the Nova fluff.
"Instead, a compromise was reached which limited the Space Marines to vessels whose
primary role was that of transport, delivery, and
suppression designed to facilitate planetary assault. Only the smallest of vessels would be permitted to act exclusively as
gunships..." Armada, page 20. (italics mine)
"Inevitably, the wrangling over interpretation of a ship’s ‘primary role’ leads to some chapters possessing rather more versatile fleets than the Imperial Navy is entirely comfortable with." Armada, Page 20. [no italics needed]
"The vessel, although larger than any genuine
gunboats likely to be in use, is relatively small by Space Marine standards, seldom carrying much more than a single squad of Space Marines... and is of little use in their favoured planetary assaults, but instead earns itself an admirable place as the foremost ship of the line when it comes to more mundane border patrols and deep space blockades." Fluff for Gladius class
"It lacks sufficient size to really fulfil the
deployment and assault roles for which Space Marine ships are primarily intended and its lance armament and speed make it a menacing
gunboat in its own right." Nova class fluff.
Again and again it comes back to 'gunboat' and 'primary role'. Not lances. Not torps. Not anti-ship weapons. The role the ship is designed for. A ship built exclusivly as a weapons carrier would qualify as a 'gunboat'. A strike cruiser with a str 2 lance would not. In fact, a Strike Cruiser, designed to be the first ship to enter a contested system, would be totally justified in having lances, as it's 'primary role' is given in several places as a first responder to a crisis.
"A typical offensive against a rebel or alien-held planet begins with the arrival of strike craft which engage and clear away defending system ships and may establish a hidden base located within an asteroid field or on a small moon if a protracted campaign is being undertaken." Armada, page 26
"Their primary function seems to be that of rapid response, reports indicating that they are invariably the first craft to arrive at a threatened planet." - Strike Cruiser fluff entry.
"Like battlebarges, strike cruisers do not represent a single class of vessel,
or specific configurations of weapons and systems, but rather represent a broad range of different Space Marine vessels used for largely similar tasks. ... Other chapters, too,
possess widely differing forms of strike cruiser, each shaped by centuries of tradition and varying doctrine amongst the chapters of the Adeptus Astartes." Armada, page 23 (italics mine)
By the logic presented by the anti-lance faction, as long as the ship has no lances, it's allowed, because it's the lances that make it a threat. This flies in the face of fluff, as clearly the prohibition is on 'pure' warships with no capability to support landings.