SNI will matter if the server offers several different web sites (different certificates) on the same IP address. In this case if the Digital Asset Link service didn’t know how to use SNI it would be shown the wrong certificate and fall.
It is also possible that DAL genuinely doesn’t trust Let’s Encrypt. This is likely to be an oversight rather than an intentional decision. In particular if DAL relies on a Java backend we know older Java versions don’t trust Let’s Encrypt. Current releases do.
Either way I’m afraid there might be nothing you can do except ask them (Google?) to fix it. I’m sure the Let’s Encrypt team would be happy to talk to them if they need any specific technical reassurance but most likely they just need to update something in their service. I know some Googlers but it’s a big company, so I doubt I can help further.