You can use AFP over TCP/IP on pre-X Mac OS to connect to a Time Capsule
Does this work?
I would have presumed that this device, being from after 10.5 launched, would have an AFP server that matches 10.5's capabilities, and to the best of my knowledge, 7.5.5-9.2.2, even with the OT/AS updates, cannot connect to 10.5 server.
I agree fully though, that MacWorld is being moderately alarmist in this article. I would bet that Time Machine/Capsule in Mac OS will hang out for a number of additional years. Heck, Mac OS 11 still connects to the AFP server in 10.4, so Apple's retaining a moderately surprising amount of compatibility there.
I've been tempted to look for a Time Capsule, myself, or an AirPort that can have a USB disk attached, for this use, but I ended up landing on a 2011 Mac mini with 10.3 on it to serve a Time Machine volume, with its internal disk backing up to an external disk, just to be super safe. (To be honest, that is arguably unnecessary, but I had it laying around.) I would be compelled to go buy one for realsies if iPhones could back up directly to a Time Capsule, but Apple would also have my $40 if iPhones could back up to a USB-attached hard disk connected through the USB+power adapter, originally meant for iPads.
Anyway....
yeah, you can use the time capsule as a file server, it can't hurt to pull it out, I just don't have a lot of optimism that it'll work for Classic Macs.
I suppose you could pull it out, set it up and see, at worst, it doesn't work and you have some options to split your time machine load, or you've got an extra access point to use for old Macs.
The other suggestion I've seen online is using a Time Capsule or an AirPort Disk for external sharing, although I forgot what I had searched to find that. You could also just do that with a USB disk on your current Time Capsule, and, if you already have a file server then you probably don't need that, per se.