https://github.com/mongodb/specifications/blob/master/source/uri-options/uri-options.rst#directconnection-uri-option-with-multiple-seeds-or-srv-uri says:

The driver MUST report an error if the directConnection=true URI option is specified with multiple seeds.

In reality:

import { MongoClient } from 'mongodb';

const url = 'mongodb://localhost:27017,localhost:27018,localhost:27019/?directConnection=true&replicaSet=replset';
const client = await MongoClient.connect(url);
console.log((await client.db('admin').command({ hello: 1 })).hosts);

prints [ 'localhost:27017', 'localhost:27018', 'localhost:27019' ] with the 4.3.0 driver. With 3.7.3, this fails as expected (“directConnection option requires exactly one host”).