While the portable version itself is not directly less secure, its still a higher risk using it compared to the standard version.
The portable version does nothing further to keep the private keys secured. Using it on an infected pc will mostly result in a loss of coins.
It really isn't a higher risk compared with standard version... Using the standard version on an infected PC will be just as bad as using the portable version.
As Thomas' post highlights, the risk is not from the app itself, it simply comes from the way it
can be used... ie. if you're actually using it in a "portable" manner... that is to say, you're running it from a USB stick
on multiple/"foreign" computers... which is not really a great idea.
However, I would suggest that an argument could be made that using the portable version on your own computer (and NEVER using it externally) is possibly MORE secure than using the installer or standard version:
- Portable version doesn't leave as many visible traces of an Electrum install on your PC... No menu items etc, anyone using your computer would need to know where to look.
- Wallet data is NOT stored in the "default" location of %AppData%\Electrum\Wallets etc... remember the "Electron Cash copying wallet files" scandal? Wouldn't have happened if you used a portable version, also any malware aimed at copying wallet files may not find wallet files etc