This is what it says on the website.
NEW! Earn 5 Tier Credits for every $1 spent on hotel room rates and resort fees when reserved directly with us.†
† Tier Credits earned are based on a single promotional day's activity. Promotional days vary by location. Check with a Caesars Rewards Center for details. One Tier Credit bonus level awarded per promotional day and does not count toward Reward Credit balance, promotional Tier Credit earnings, or qualifying for another bonus. Tier Credits earned from a promotion or multiplier do not count towards earning a Tier Credit bonus.
I think it's pretty clear that if the TC bonus applies to spend at all it would not include the 5X multiplier.