Yes, solicitor seems to be mixing two things up:
1) if the transfer availed of dwelling house exemption i.e. a gift to you of the house from your step parent and you occupied the house for the 3 years up to the date of the gift then it is exempt from CAT so long as you continue to occupy the house as your main residence for the next 6 years
2) If the transfer did not avail of dwelling house exemption and used up your threshol, then you can sell at any time.
Generally its preferable to avail of dwelling house exemption if at all possible - this preserves your CAT category A threshold in case of other gifts/inheritances