It would be fair to call Speaker Pelosi’s letter seditious, both technically and symbolically. A bit of an exageration, perhaps, but fair.
The president unquestionably had constitutional authority to kill Soleimani. The Speaker’s intention to constrain that authority is an attempt to subvert the constitutional order.
The Speaker of course has co-equal authority to pursue the intentions her letter describes. Ultimately, however, the only Congressional remedy for limiting the powers of the presidency is impeachment. And good luck trying to make a case for impeachment out of killing a known enemy combatant.
In other words, Pelosi’s claim of “oversight responsibilities” is an egregious overreach. Sedition, in fact.