Well, he does disagree a little bit :)
The language of the WA and the legislation that gives it effect does state that future legislation will be subject to the provisions of the WA and be interpreted in accordance with the WA. The WA legislation is the product of a treaty between two parties so the principle of not allowing unilateral legislation to override it is sound.
In practice a Govt could take the political decision to ignore this and specifically instruct the courts to set aside all aspects of an international agreement if this was challenged, but then once you do that, it allows other countries to do the same with respect to international agreements they sign with us. Spain with Gibraltar for example.
Personally I don‘t see the Govt doing this, make a lot of noise, get as much wriggle room as possible under the protocols, yeah but disavow entirely? No. On the other hand the damage has been done. German news reports are all about how ‘unreliable‘ the UK Govt is, how ‘untrustworthy’ we are. That perception will be difficult to shake off.