You are dead on!
The only way to send TTs with the stock HM151 is via the memories. You can replace the stock mic with a different Icom unit, and do it, but then you lose the other stuff. Personally, if I needed TTs for what ever reason, I'd replace the mic. Probably replace it anyway, as it is just to busy. I'm used to using the memories to select the bands, and modes, so the simpler the mic the better.
There are two other drawbacks you might not be aware of. First, the band data has been disabled. If you require it, you have to open the radio and solder a bridge. Why on earth Icom did this, I'll never know, as their current and legacy amps all require it.
Secondly, there is no audio out of the mic jack. Pin 3 is now a mic sense to tell the radio you are using the stock HM151, and not a TT mic for example.
They tried to make up for the lack of audio by moving the headphone jack to the right upper side of the radio. In the correct spot if you want the cable interfering with the VFO knob! It should have been located on the bottom, preferably next to the mic jack. Poor ergonomics if you ask me.
What I am most miffed at is the lack of a remote kit, and it appears now it will be late March to early April before they are available. This recent delay is the fourth or fifth time Icom has moved the availability date out. Makes you wonder what else they didn't do.