Type checking ESM with mapped package fails to see types within package

[MasterKale]

My project uses @hexagon/base64, which cross-publishes to deno.land as b64. I'm experimenting with migrating my Node library to use dnt so I can start cross-publishing as well, and so I updated my import to point to deno.land:

import base64 from "https://deno.land/x/b64@1.1.27/src/base64.js";

Which I then map to the NPM equivalent in my dnt build script:

mappings: {
  'https://deno.land/x/b64@1.1.27/src/base64.js': {
    name: '@hexagon/base64',
    version: '^1.1.25'
  }
},

When I run the build script, the export maps over as expected:

import base64 from "@hexagon/base64";

BUT dnt fails to build when it tries to type check the ESM it just output:

deno task build
Task build deno run -A build_npm.ts 0.0.1
[dnt] Transforming...
[dnt] Running npm install...

added 2 packages, and audited 3 packages in 849ms

found 0 vulnerabilities
[dnt] Building project...
[dnt] Type checking ESM...
src/helpers/iso/isoBase64URL.ts:1:20 - error TS7016: Could not find a declaration file for module '@hexagon/base64'. '/Users/matt/Developer/simplewebauthn/packages/server/npm/node_modules/@hexagon/base64/dist/base64.cjs' implicitly has an 'any' type.
  Try `npm i --save-dev @types/hexagon__base64` if it exists or add a new declaration (.d.ts) file containing `declare module '@hexagon/base64';`

1 import base64 from "@hexagon/base64";
                     ~~~~~~~~~~~~~~~~~

error: Uncaught (in promise) Error: Had 1 diagnostics.
          throw new Error(`Had ${diagnostics.length} diagnostics.`);
                ^
    at getProgramAndMaybeTypeCheck (https://deno.land/x/dnt@0.38.0/mod.ts:456:17)
    at build (https://deno.land/x/dnt@0.38.0/mod.ts:345:17)
    at eventLoopTick (ext:core/01_core.js:183:11)
    at async file:///Users/matt/Developer/simplewebauthn/packages/server/build_npm.ts:7:1

The @hexagon/base64 package includes its own .d.ts file, though, that has all the type declarations:

It doesn't seem possible to get dnt to see these types. My project currently gets TypeScript to see these types by including "./node_modules/@types" in tsconfig.json's typeRoots (which, I know, doesn't really make sense since @hexagon/base64's types aren't actually in node_modules/@types/ but it works 🤷‍♂️), but typeRoots isn't a valid value for dnt's compilerOptions:

I tried adding /// <reference types="https://deno.land/x/b64@1.1.27/types/base64.d.ts" /> above my deno.land import in the source file, but it just gets stripped out by dnt so that doesn't seem to be an option either.

Any idea how I can get ESM type checking to see this package's types?