I understand that they were in 13 a ton by the end of the year, and it was hugely successful but it was about 35% overall for the season; and, I believe that was twice as often as the next highest team ... That's Extreme.
Another factor to consider, at the end of the regular season, was the absence of Davante Adams due to injury. With only Mumpfield, Whittington and Smith opposite Nacua (who they may have been trying to keep fresh for the upcoming playoffs) two-WR sets may not have been an attractive option.
McVay was clear about being excited for (his words) 'the next iteration of 13-personnel' but the question remains ... How often will it be employed. By-the-way, if the Rams play 13-personnel two-thirds of the time for the entire season, the Rams may well have enough at Wide Receiver right now.
Personally, I hope the Rams have enough talent and depth at WR to play 11-personnel when McVay wants because they have had tremendous success with that formation for most of his tenure with the Team.