You neglected to quote the most relevant part of the article which answers your own question regarding Monero:
Proton Wallet is in a strange position. I’ve spoken to a few sources who suggest that privacy features like CoinJoin, which can mix Bitcoin in order to better anonymize transactions, were intended to be included at launch. The crackdown on the ill-fated Samouri Wallet project by U.S. authorities last April certainly put a damper on privacy in the Bitcoin space, and likely made Proton wary of introducing such features to the public.
Proton suggests this themselves, stating on their website:
“Coinjoin is considered the best solution for improving blockchain privacy. It works by mixing your BTC with other users’ BTC in a collaborative self-custodial transaction where you get back the same amount of BTC that you put in but on a different address that cannot be easily linked to your previous address. However, in 2024, in what many consider to be a regulatory overreach and attack on privacy, some of these Coinjoin services have been declared illegal in the US and EU. The future of financial privacy may therefore be decided by ongoing litigation in the next decade and privacy advocates should support these efforts.”
This situation likely soured Proton on other privacy-friendly cryptocurrencies like Monero as well. I get it, financial privacy is an extremely challenging task for any company to take on. We can’t expect Proton to take on the risk of offering a completely anonymous payment service in the current legal climate, but it begs the question: why enter the financial space at all?
While not particularly revolutionary, the fact that they provide a unique HD wallet address every time you receive funds through your same email address does provide additional privacy as no one can see your previous transactions. Even when those are rolled up together later it does make it harder to associate an exact total balance with you. If you used your wallet for smaller spending rather than making a single large send from it, that makes it harder still.
Sure, I would have liked them to add Monero too, but it gets thorny when you’re a regulated company dealing with that.