This is true; however, I don’t recommend it.
In my case, I was racing professionally and on contract. I also wasn’t injured such that riding/racing would exacerbate the injury. It just hurt a lot.
Most broken bones take that standard 6-8 weeks as you mentioned. With ribs, it’s weird, because you’re in a world of pain and then suddenly one day you wake up and it doesn’t hurt anymore. The progress doesn’t feel linear, and if you’re healing properly, it probably won’t take a full 8 weeks to be pain free.
Your main goal is consistency. This is as much about stewarding your own intrinsic motivation as it is about recovering physically and staying healthy. Please keep that in mind. Assuming you can power through the pain without doing more damage, you could certainly get militant about your training and will yourself through daily workouts. However, this takes an enormous toll on your reserves of willpower and motivation; resources you’ll need later in the season when training becomes more important, or more crucially, in the middle of your Ironman! Humans experience pain not only as a physical sensation but as inextricably tied to emotions - often anxiety or fear, so don’t underestimate the mental and emotional toll of pushing through pain (especially broken rib pain). Remember that you’re just beginning what will be a very long build toward your goal event (September is a long way away!) and that you’ll need to pace your efforts in terms of physical stress as well as mental and emotional load. You’re the freshest (mentally, emotionally, and physically) that you’ll be through this whole build, so not unlike the start of a long race, you don’t want to get over confident about how great you feel right now (mentally and emotionally in the bigger picture of your training plan - not talking about the rib here ) and burn through a bunch of matches too early. I’d say that since you have so much time to your goal event, and since you’re not obligated to ride through this to make a living, err on the side of caution and give yourself plenty of time to heal and recover without burning more energy on this injury than necessary.
If Z2 work feels okay - go for it, especially if it’s helping you feel recharged rather than drained. I found that taping my rib helped a lot - even just a big ace bandage wrapped around the rib cage for compression helped enormously with the pain, and thus with RPE and general mental/emotional stress components of training stress. I recommend staying on the tops/hoods. You can always work on technical aspects of your pedal stroke technique, or try different mantras or other mental exercises to test what mental strategies might help you in training once you’re back to 100%.
@DarthShivious makes an excellent point about avoiding infection due to shallow breathing. Rib mobility is HUGE for cyclists as it affects lung capacity and generally gets worse with age. Once you’re healed, it’s probably wise to consult a PT about how to regain lost mobility and/or optimize your mobility for the season. Mobility of your ribs, scapulas, shoulders, diaphragm, lats, etc, all strongly affect your lung capacity and will ALL be strongly affected by the broken rib. Good to get on that as early as possible!
I always recommend the website Injured Athlete’s Toolbox to any injured athletes I know. It’s wonderful for helping you feel seen/heard re: the emotional fallout of injuries like these and can be a real resource. Heidi is available for one-on-one coaching, but I’ve even found simply reading her website helps.
And, if it’s of interest, here’s a piece I wrote about the frustration of coming back from injury (and the importance of healing properly) after I broke my pelvis. The inspiration for the piece comes from a quote I read somewhere: “When in a hurry, take the long, sure path.”
You’ve got plenty more training ahead. For now, look after your physical, mental, and emotional wellbeing. If that includes some light training, great. If not, that’s okay too. Sometimes it’s worth taking two steps back to get a running start. You’ll make much faster progress once healed if you’re mentally fresh and chomping at the bit when it’s time to get going again!