No they're not
All you do is tell your DD that you were a bit cross and you shouldn't have said that. Next time you have a situation (ie. in 10 minutes!) choose something smaller that you can follow through on and are willing to follow through on. Then you say 'dd I'll do x...and I mean it!!'.
She'll either believe you, in which case you didn't do any harm in going back on your threat earlier, or she won't, and she'll realise you really do mean it when you follow through, in which case she'll think 'ok, mum meant it this time!'
You won't break her by being inconsistent occasionally, honestly you won't. I know it's so alien when you have a DS who needs absolute consistency at all times to learn, but NT children are freakishly adaptable and able to work out the subtleties of situations.
DD3 is incredible at sussing out what's going on. There are times when I am at the end of my tether cross, and I shout at her (not ideal). She responds, very sensibly, by doing whatever I've told her to do at a fast rate of knots. Then there are times like the other day:
I wanted just 4 minutes to read something. DD3 was not going to let me. She was coming in my bedroom (DH was getting ready for work & pottering with them to allow me a few minutes). I asked her, then told her, then ordered her, then YELLED at her to just 'shut my door, shut my door, shut my door'
. Her response?
"Mummy...."
WHATTTTT??????
"Do you want me to shut your door?
"
Now, how did she know that this time she could win me over? They're dead clever :)