The standard for SSL/ TLS says that the server must send a list of certificates that form one single trust chain in order. In the screenshot you included they’ve done that, and it forms the path “Path #2: Trusted”. The “Path #1 Trusted” just stops part way along that route and diverts to a signed root that SSLLabs know about. But until that point every certificate in the path is one of the same ones from the longer path below.
To provide both the ISRG-signed intermediate and the DST-signed intermediate would mean a fork in the presented chain, it would no longer form just one chain, to a root, but two possible chains. This may cause compatibility problems.
A server might spot that this isn’t a standards compliant chain and refuse to serve it.
A client might spot the same thing and reject it, even if it would have accepted one, or even both of the presented chains on their own.