Yes it is, because you can't backup the installer and transfer it to a different rig without internet connection, or install it another time, when the internet is down. Your argument is based on the assumption that everyone immediately installs their games after downloading (and maybe does not even see a need for deinstalling them, ever). And that everyone is playing on the same computer from which they have access to the internet.
But in the end it doesn't really matter how you define DRM for yourself because contrary to popular belief, most people who argue against DRM are not just allergic to the three letters but have real issues with the mechanisms behind them, regardless of what they're called. ;)
You can backup and restore installed games on Steam, however having not used that feature myself I'm unaware if you need to be on-line or not during the restoration process.
I would personally love the ability to legitimately create an off-line installer for games I've purchased on Steam, for the unlikely event that Steam becomes unavailable in the future, however the peculiarities of Steam's particular distribution method, however flawed they are, do not themselves constitute DRM.
I have no fondness of the drawbacks that arise from the fact that Steam provides no option of having detached installer for the games you purchase, particularly since if you lose access to your Steam account, you lose access to all the games you've legally purchased on there, with no way of legitimately recovering them. However when arguing against something, it is important to be factually correct about the things you're arguing against.
Save for the technical differences in the protocols used and the interface provided, there is no difference between Steam's distribution method, and GoG's, the difference comes in what is distributed. GoG distributes pre-packaged installers for games, where as Steam distributes game files which are 'installed' on the fly.
My argument isn't in defence of the flaws of Steam. My argument is about making distinctions between what is DRM, and what is not, and when I say DRM, I'm not talking about the three letters, I'm talking about the practice of "rights management", the active restriction of how media can be used by legitimate customers. Something which I strongly object to, along with other restrictive and obnoxious practices used by publishers, such as EULAs.