At this time, the use of oidcConformant: true in Lock is not yet documented and as such you should not use it as the lack of documentation also implies the possibility of some behavioral changes. To my knowledge, documentation for this should be coming soon, but don’t take this as a promise. In relation to the specific error I would assume that you’re using username/password authentication with Lock and that explains the limitation you observed when used with MFA; this limitation may not exist in the final documented version.
For now, if you want to leverage user authentication and API authorization in association with MFA my recommendation would be for you to use Auth0.js v8, in particular, the webAuth.authorize method.
As an additional note, the reply in the post you originally mentioned was aimed at support of MFA in the resource owner password endpoint and not in Lock itself. See this for additional information on this; however, for SPA’s the recommendation would still be the one above (Auth0.js v8 with authorize method).