L4S (or at least, the dualq implementation) uses a shallow buffer. You can't get 1000ms of latency if your buffer is only 15ms. Although the title claims there's no way round latency spikes, their paper [1] explores the case where latency spikes are traded off against allowing packet loss, which is what you're going to get if you fill a shallow buffer. For real-time traffic, packet dropping is usually preferable.

[1] https://arxiv.org/pdf/2111.00488