I'll point out, the Lion's Guard is not the Sindarian Diktat faction, it is a sub-faction without any markets or commission. So the Sindarian Diktat itself is not getting a nerf. It's been stated by Alex the only ship used by the Lion's Guard you'll be able to buy with a Sindarian Diktat commission is the Executor, which is actually used by the standard Diktat forces, since it's their capital as well. Given there's no direct equivalent to the Executor because of the mod slots (and maybe ship system - that is unclear to me), it is a hard call declare it's balance, since it is a different ship from everything else in the game. As for the built in Solar Shielding on it, there are plenty of ships with built in hull mods that their OP allotment likely takes into account but are highly situational. Odyssey has High Resolution Sensors, for example.
The d-mod on the capital can be restored, but is admittedly a financial consideration (or a skill tree consideration) not needed when purchasing pristine ships from other commissions. On the other hand, you can't get that particular capital from other commissions, since it is a single faction ship.
The Sindarian Diktat itself will provide you with Eagles, Falcons, Hammerheads, and so on just as good as the Persean League provides. They just don't come with the purple paint job. Pretty sure you won't see Lion's Guard inspections, so the only time you'll interact with them is around Sindaria.
As for "horrible" D-mod, if the effects are minor as Alex implies, like 5% effects, it might be one of the best d-mods for Derelict Contingent, assuming you're going down that route. We'll have to see the final numbers. Although, as far as I understand it, the only way to get Lion's Guard only ships is to salvage them, which means they'll likely come with other d-mods that you'll want to restore anyways in more typical play throughs.
Overall, the Lion's Guard exists to fulfill a story function as opposed to a game balance or new combat option mechanic. The update isn't saying these are necessarily stupid people (they are likely bad in the sense of morality), but it is representing a cult of personality dictatorship. Doesn't matter if the people are fundamentally dumb or not, it is quite possible for the closest people to the dictator to evolve into yes-men or dead-men, which is exactly the point the Lion's Guard is making. It's a bunch of yes-men military flunkies which are closest to the Supreme Executor, while the rest of the military carries on competently.
Also, Alex has implied he's likely going to change the built-in at full price Solar Shielding, so that may not be an issue in the fully released version.