I don't claim to have a deep understanding of Northern Ireland politics.
Same here, but I'll attempt an answer anyway. So, your quote comes directly from Sinn Fein's website, here. The title of that article is as follows:
British Government has abandoned any pretence of impartiality - Michelle O’Neill MLA
From that and your quote it's clear that Sinn Fein does not want direct rule. The article ends with the following:
“The Irish Government has a responsibility as a co-guarantor of the Good Friday Agreement to oppose any attempt by the British Government to act unilaterally break joint international agreements.”
Now we know where to look, the grave implication seems to be that direct rule might be in conflict with the GFA.
Looking into the combination of 'GFA' and 'direct rule', we find an opinion peace by mister Fitzgerald (apparently, a former football player) in The Irish News. In that piece, he wrote the following:
Any measures by London to bring back direct rule could legally be construed as invalidating the GFA. The GFA is a contractual and constitutional document which is designed to replace direct rule. Because it has been frustrated in its workings on power sharing, it could even now be discharged notwithstanding the declared imposition of direct rule by the secretary of state. Given the long multiple suspensions of the GFA, anybody could potentially claim that devolution is no longer in effect in Northern Ireland. It is only in effect if there is power sharing which has now become frustrated. Legally one could also argue that direct rule is already in effect by default – in default of the GFA.
For obvious reasons, the GFA is important for peace in Northern Ireland, and the suggestion that the peace will be ended is very worrisome.