It can be interpreted both ways: "I cannot recommend X too highly (because they suck)" vs "I cannot recommend X too highly (because whatever praise I give will be inadequate)"
If praise is the intent, it would be phrased as "... cannot recommend X highly enough"
I disagree.
Not every opinion is equally valid
My take away from this is that any positive thing written in a letter of recommendation can be read as sarcasm by an English speaker.
I think there's some deeper issue with the language/culture here.
Every language has turns of phrase that are not necessarily intuitive to non-native speakers.