Running yarn dtslint tests/types-dtslint results in a syntax error for me.
/Users/[...]/.dts/typescript-installs/4.3/node_modules/typescript/lib/typescript.js:139
for (let i = startIndex ?? 0; i < array.length; i++) {
Which suggests dtslint does not support nullish coalescing?
Have tried with dtslint@4.0.7 and latest dtslint@4.2.1. The repo has been archived and the project rolled into definitely typed. Not sure where that leaves us.