I think the UK establishment is playing with fire here. Truss is a red herring: she's tactless, it's her thing. But her comments about ignoring Nicola Sturgeon are symptomatic of a serious, ongoing problem.
Johnson has said worse, as did Mordaunt and Badenoch in the larger leadership contest. Sunak has publically changed his position from supporting self-determination to opposing it in principle. No one in the Con leadership race has respected the UN-recognised principle of self-determination of peoples. Theresa May's "now is not the time" was controversial in 2017, but looks positively liberal by today's Unionist standards.
Labour's no better. In their last leadership race Lisa Nandy suggested that the UK should use Spain as a model of how to deal with Scottish separatism (!!!!!!!) and Jess Phillips replied to a question about Scottish separatism on national TV by throwing up her hands and exclaiming "I don't know what they're doing up there!"
I understand Truss's annoyance with Sturgeon as an individual and as a politican. Some Scots share it, both those who support independence in principle or in reality and those who pray it doesn't happen. But, crucially, the next UK PM can't just ignore Sturgeon. She IS FM. Her party won and holds the largest number of seats at Holyrood, and - as Stugeon herself will point out if she hasn't already - she was also (re)elected as FM by a majority of MSPs of all parties.
Scots do have the right to self-determination, as do all national groups. People of good faith and of many parties and none want independence and work for it (or can be persuaded to do). Many who don't want independence now (and perhaps not ever) still respect the principle that Scotland has the right to decide.
A sensible unionist leader would make the argument that there is not enough appetite among Scottish voters for a referendum to result in a vote to separate, and therefore the expense and disruption of a second referendum is not justified now but can be considered if circumstances change. And in the meantime, make the positive case for the union - NOT as a "one size fits all", the UK does what England wants because England has a relatively huge population so "democratically" gets to choose. Support the FM, the Scottish Government and Scottish Parliament in devolved matters and listen to Scottish MPs (of all parties) in reserved matters.
"What we need to do is show the people of Scotland, Northern Ireland and Wales what we're delivering for them and making sure that all of our government policies apply right across the United Kingdom."
This, from Truss, is decent in principle but stated badly. It sounds like she either doesn't understand the Scottish devolution settlement or wants to erode or reverse it. That will go down like a lump of sick in Scotland, where only the most headbanging Tories (and the Scottish politics experts telecommuting from Novosibirsk and Khavarovsk) oppose devolution and want to abolish the Scottish Parliament.
After what we've seen over the last eight years, I don't rule out the idea that this is all political theatre. The UK PM blusters against Scottish self-determination, stirring opposition in Scotland. When the inevitable UK election comes more Scots vote for parties that clearly priorities the needs of their constituents in Scotland (which at the moment, logically, are also independence-supporting parties). Sturgeon announces that the UK election will be a plebescite for a second independence referendum; if pro-indy parties gain a majority of the Scottish seats in the HOC, independence will be pursued with or without Westminster. (Ian Blackford already said this in the HoC: Scots cannot vote for either Cons or Labour as both fundamentally oppose Scotland's interests regarding a desired future in the EU and/or the European Single Market).
Imagine for a moment that a second referendum is held in 2023. One likely scenario: all sides behave controversially, turnout is lower than ideal, "Yes" narrowly loses. Sturgeon complains about the conduct of the Unionist parties/"No" campaign and strengthens her party's position in both Scotland and in the UK Parliament, but now she's completely off the hook for delivering independence, with its inevitabllity of shoving her party out of power. And the UK gets to "keep" Scotland.
The UK establishment may be 100% assured that independence is impossible with Sturgeon in place, but even Tito and Mugabe (still waiting, Putin) eventually were replaced. Power in Scotland belongs not to any government, parliament, or monarch, and certainly not to any party or cause or individual. Power in Scotland belongs to the Scottish people. Always did, always will. Be careful who you piss off.