The EmailTemplate metadata type does not support wildcard retrieval[0] - in package.xml or using the CLI. You need to specify each individual EmailTemplate you want to retrieve. E.g. sfdx force:source:retrieve -m EmailTemplate
won't work, but sfdx force:source:retrieve -m EmailTemplate:folder/MyEmailTemplate
should .
[0] https://developer.salesforce.com/docs/atlas.en-us.api_meta.meta/api_meta/meta_emailtemplate.htm#!