Correction: that you can control the IP space advertised to Let's Encrypt. A BGP exploit would result in getting valid certs for someone else's domain/host. It's almost trivial to exploit BGP, which is why PKI is so important... so it should actually be incredibly difficult to get a cert.

If someone is hijacking BGP to MITM the world, how are the old methods any better?

If you could hijack BGP you could just as easily inject routes to intercept DNS requests as well.

This is why ISPs use tools like RPKI validation of received routes, and things like the ARIN, RADB and RIPE route-servers.

Almost nobody uses RPKI: https://rpki-monitor.antd.nist.gov/