Firstly, the relevant paragraph 1 of Article 17 of the Treaty is subject to the 'savings clause', so the IRS can tax it whatever the treaty says.
A rollover from a conventional IRA to a ROTH IRA is normally treated as a taxable distribution from the IRA with reinvestment into the ROTH IRA within 60 days. Your argument seems to rely on the Treaty statement that a rollover from one account to another is not treated as a taxable distributiion, which clearly contradicts the intended method of rollover under normal circumstances.
From:
https://www.irs.gov/Retirement-Plans/Retirement-Plans-FAQs-regarding-IRAs-Rollovers-and-Roth-Conversions"How do I convert my traditional IRA to a Roth IRA?
You can convert your traditional IRA to a Roth IRA by:
Rollover – You receive a distribution from a traditional IRA and contribute it to a Roth IRA within 60 days after the distribution (the distribution check is payable to you);
Trustee-to-trustee transfer – You tell the financial institution holding your traditional IRA assets to transfer an amount directly to the trustee of your Roth IRA at a different financial institution (the distributing trustee may achieve this by issuing you a check payable to the new trustee);
Same trustee transfer – If your traditional and Roth IRAs are maintained at the same financial institution, you can tell the trustee to transfer an amount from your traditional IRA to your Roth IRA.
A conversion to a Roth IRA results in taxation of any untaxed amounts in the traditional IRA. The conversion is reported on Form 8606, Nondeductible IRAs. See Publication 590-A, Contributions to Individual Retirement Arrangements (IRAs), for more information."
Under Article 17, at the end of Paragraph 1 on page 63 of the Technical Explanation to the treaty it says:
"Similarly, if the distribution were not subject to tax when it was “rolled over” into another U.S. IRA (but not, for example, to a U.K. pension scheme), then the distribution would be exempt from tax in the United Kingdom." But under normal circumstances, a conventional IRA is subject to tax in the US when it is rolled over, as stated at the end of the prior quote, and so i suppose it would be subject to tax in the UK as a distribution even if it escapes tax in the UK due to the treaty provision. Note that it is not tax-free in the US under the treaty; it is subject to a witholding rate of zero so that it is fully taxable in the UK without any credits being available.
If you google for this technical explanation, you will only find the defunct one from 1970s. Neither the UK nor US authorities currently makes it available for public access it seems, so that ordinary guys like us can't read it (if it still exists). I think one or two tax firms have paraphrased on their websites as as if it were their own advice. I guess I must have struck lucky just after it was published and before it was removed. I wonder if anybody has anything to say about that !?