The offer does not say anything about reserving the right to...

The offer does not say anything about reserving the right to increase the amount if oversubscribed. In fact the statement in bold below is the opposite.

"The SPP provides Eligible Shareholders with an opportunity to buy a parcel of shares in the Company with no brokerage or transaction costs, irrespective of their existing shareholdings, up to a maximum of $30,000.
The offer price is $1.55 per share (Offer Price), which is the same as the issue price under the Placement.
The minimum application amount is $2,500 and the maximum application amount is $30,000. The Offer Price
represents a:
• 15.47% discount to the Volume Weighted Average Price of $1.83 during the 10 trading day period
leading up to the Announcement Date; and
• 10.40% discount to close at 25 February 2021 (being the last trading day before details of the SPP
were announced) of $1.73.
If the total value of applications received under the SPP is greater than $3 million, the Company will scale back the number of New Shares that will be allotted under the SPP and you may be allocated fewer New Shares than the number for which you applied. In such a case, you will receive a proportionate refund of your application monies.
Participation in the SPP is optional and the right to participate is not transferrable. The offer to participate in the SPP closes at 5:00pm (AEDT) on 29 March 2021.
Full details on the SPP and how to participate are contained in the attached documents.
In order to apply for a parcel of shares, please read the attached documents carefully and follow the instructions on the enclosed personalised application form."

In fact the proposed securities issue notice to the ASX limits the amount of shares for the SPP to $3m worth. That may be because the total percentage they can issue without Shareholder approval is limited. I am not 100% sure on what that figure is without further digging though...