But it's possibly not the only way independence could be achieved legally. Maggie Thatcher used to hold that if Scotland returned a majority of SNP MPs, that would be enough to secure independence. Times have changed since then (the SNP currently has 48 out of 59 Scottish MPs).
The SNP's current plan is to clear the way for a second referendum with the co-operation of the Westminster government, under Section 30 of the Scotland Act 1998, which is what secured the first independence referendum. This is unlikely to be forthcoming any time soon, if at all, in which case we're likely to enter uncharted legal territory along the way, involving the Act of Union and later legislation. Andrew Tickell, one of Scotland's brighter legal brains and sympathetic to independence, feels that legal avenues offer no clear path forward if a Section 30 order isn't granted. The headline to the linked article is more categorical than he's prepared to be, but basically concerted political pressure is what he sees as the way forward in the absence of a Section 30 order. This may involve action in various courts, but it may end up offering more of a campaigning tool than an ultimate remedy.
Scotland has a great deal of goodwill in the EU, having undertaken a concerted campaign of quiet diplomacy since the 2014 independence referendum, which may help as the UK's negotiations unfold over the next few years. It's a far cry from the first independence referendum, when the threat of Scotland losing its place in the EU was persuasive for many people and the EU was generally hostile to the idea of Scottish independence, and the boot is now on the other foot, with various EU officials and politicians saying on the record that Scotland would be welcomed with open arms if it became independent and accession could happen very quickly. Whether that would be the best option for Scotland if it did gain independence is another question, as support for EU membership isn't universal even among independence supporters, so it might end up that an EFTA-type arrangement or whatever would suit it better, at least in the short to medium term.
The path to devolution which set up the Scottish Assembly (later the Scottish Parliament) was far from clear, too, and after some 20 years of wrangling it took some clever blindsiding of the UK government by Scottish politicians' recourse to international law courts and international political pressure (threats of legal action even) from the EU before it was granted. There are some very smart politicians and lawyers in the independence movement, so we'll have to wait to see what they can come up with in addition to popular pressure.
Polls since the Brexit referendum have shown that many in the UK aren't that bothered about the Union, whether it involves Scotland or Northern Ireland. That may change if Johnson sees it as a useful rallying cry/distraction from the troubles ahead in exiting the EU. It would be a tortuous argument given the government's repeated claims that Scotland's economic welfare is dependent on the rest of the UK (which I don't intend arguing in detail here as it gets very technical, but basically it's untrue), and many of Johnson's supporters may feel they don't want to "bail out" Scotland any more.
The last independence referendum was agreed at a time when independence was only polling in the high 20s percent, and the closeness of the final result gave David Cameron conniptions. Currently it's more like 50:50, even before last week's general election result, so that may have some bearing on whether the UK government feels it wants to risk going ahead with another referendum. On the other hand, there are Tory voters but precious few Tory MPs in Scotland, so Johnson may feel like it wouldn't be the end of the world for his and future Tory governments if independence happened.
Meanwhile, noises from Johnson et al. so far indicate that they intend to pare back, if not strip, the Scottish Government's powers, which is likely to inflame feelings even more.
Whatever happens, I'm bracing for a bumpy ride.
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