I don't buy inventing a photon torpedo is particularly hard. Yes, humans didn't have them in 2151, but they were barely warp 5 capable back then. Warp capable at all for less than a century, even. They probably would have stumbled across the idea sooner or later
...Which is what it
means that it's difficult to invent a photon torpedo. And it's "later" if they don't go exploring and bothering Sleeping Dogs. Which they apparently don't do as a rule.
seeing as they have such a penchant for weapons.
What penchant? They had little in the way of weapons. They just fought a lot, which generally goes with not having weapons, or else the fighting would lead into a result.
What we might be seeing here is a general Satarran inability to invent and/or manufacture weapons, possibly a result of all the fighting.
It's also unlikely the Satarrans were generally advanced in weapons tech.
That, too. Which in this case means they were below the Photon Torpedo Level by quite a bit and had no way of gathering XP fast.
Had they been, all they'd have to do is steal the concept of a photon torpedo and start manufacturing themselves.
Which is probably what they were doing here. Except they'd skip the manufacturing part because they could win the war without it already, with much less effort.
As the general technology has been around for centuries and shared by several races, the details could hardly be a highly classified military secret.
Not only does it appear that it's difficult to invent a photon torpedo, it's also hellishly difficult to make one. The Maquis were credibly framed for the theft of empty photon warheads in "Tribunal", even though they had replicators and demonstrably could steal other things from Starfleet, things that might attract less attention. Presumably mere knowhow doesn't get you far in your quest for photon torpedoes, then.
Getting their own, or even simply buying them would have been far more effective and less risky in that case.
We see some arms trading in DS9, but the deals go south more often than north, even with seemingly streetwise folks such as the Maquis at it. There doesn't seem to be anything simple about buying that sort of stuff, as opposed to buying, say, random infantry weapons or used spacecraft.
It seems to me they really wanted the ship to make that devastating blow - it probably was not just about those photon torpedoes, but also about superior shields, and superior tactical defense systems, etcetera.
In short, one of those if-you-douse-him-in-gasoline-he's-warm-for-the-rest-of-his-life deals: no point in actually acquiring the weapons when merely using them once gets you there.
Although we might also suspect they wanted the Federation to destroy the Lysians, for deniability. As in, merely destroying the Lysian command center would not win the war, even if it turned the fighting into a guerrilla action rather than a series of pitched battles between fleets - but making the Lysians hate the Feds would stop the fighting, when it was in connection with the Lysians being dealt a weakening blow.
Timo Saloniemi