I am really struggling to find a source for this.
The closest I got was their Wallet Twitter account responding to a user teasing a statement on shitcoins out of them, where they only took the word from said user.
To me it is highly questionable that they meant to include XMR in that blanket statement, as the parent tweet appears to provoke a response toward actual shitcoins.
Please provide me with a source for another quote, as I am genuinely interested.
Edit: I have further investigated this and hit nothing new, but a more likely origin of the narrative:
Techlore’s first impression after the announcement of Proton Wallet.
Here is the video with a timestamp (@04:34), where he addresses the tweet I already found.
Unfortunately, it seems he misses the context of Proton’s tweet, which again, was taken from the tweet of a user they replied to.
He shows Proton’s blog post, though, where they used “altcoins”, providing more evidence that “shitcoin” was not their original wording or attitude toward Monero.
Proton also carefully commented about their consideration of altcoin adoption on Reddit at that time.
As far as I’m concerned, since I can not find any more evidence to the “shitcoin” claim, I consider this false until further evidence comes up.