It sounds like they are probably referring to the processing power of your firewall, rather than any specific compatibility.
Your new APs are very likely capable of carrying significantly more traffic than any they are replacing. How much of that extra capacity your wifi client base can realistically make use of will depend on your device mix and how they are used (what apps etc). You have to think that those devices will become more capable, over the years though, as they're replaced.
Moreover, you may well be (more) constrained by your available Internet uplink, beyond your firewall. If your link is just 100 Mbps or so, how will a Gig-capable firewall help? That having been said, your Internet pipe will probably be upgraded, within the lifespan of your APs and a replacement firewall anyway. I try to think about your whole traffi-flows and any bottlenecks along the way.
Bear in mind too that you needn't necessarily upgrade the existing Fortigate with a new Fortigate; what about an MX? 😏
In this case, the traffic is fairly light relative to the number of AP's in use. The buildings are old with thick concrete/stone walls which is why we have so many APs. There aren't actually all that many users. Since budgeting is an issue, I think I might upgrade just the APs this year and put off the FW upgrade to next year.
We have a number of sites with FortiGate FW and Meraki, Where we run FGT 30E's with MR56 AP's, albeit these are being upgraded in due course to 100F's but as it stands now we don't see any issues reported where bandwidth is concerned.
Our user counts for these sites are 60-80 users.
Your 100D will be just fine until you are ready to upgrade but do take advantage of the Fortigate upgrade program where they will exchange a 100D/E for a 100F at a significant discount with licence porting.
Not sure about the MX firewalls though we have 1 but i find the FortiGate's to be much more feature rich 🙂